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It could be that something went viral that caused your site to rank higher for a limited time.

According to that link it has not been updated since 2013/2/4

http://www.hobo-web.co.uk/google-pr-update/

I use Bing search API,

By the way, you want to change from GET to POST, not the other way around.

Use a canonical tag would be the easiest

PR as you are talking about is only updated every now and again, http://www.hobo-web.co.uk/google-pr-update/

If there is good links pointing to the domain?

what if there is bad links pointing to the domain?

Why was the domain abandoned?

Google is also a domain register, so they know that a domain has changed ownership. it has been said that links from the old owner are discounted when it expires or changes ownership, I don't know if this is true, but it is something else to consider.

The problem with PR leaks is that they are scalable, If you are losing 10%, then you get some quality links, 10% of them will be wasted, every effort you do in the future will be discounted by 10%.

There are ways to fix all these problems, for example I would make a search to be POST and not GET so that links to search pages can not be made and therefor search pages will not get indexed.

We work so hard to get good links, why waste them when you do?

as long as no one is linking to the search pages including internal links.

It that's is the quality of the links, then remove them. At best they are not helping you, at worst they are looking like spam.

I am a programmer by trade and SEO second, I build new sites and my experiences lately is that you can rank quite well without many links or even none. I truly believe only good links are worth having most links either do nothing for you or hurt you.

Chris is correct I have seen this before where it counts line breaks before and after text.

to fit in with all search engines you should keep to 65.

Another thing to consider, title tags have so much power, that power is divided between the keywords. If you have a long title not so much power is given to each keyword.

If your parents leave 1 million dollars in their will, you will be a rich man, but if you have 10 brothers and sisters then not so rich.

You should read this to the website owners

Minimize similar content: If you have many pages that are similar, consider expanding each page or consolidating the pages into one. For instance, if you have a travel site with separate pages for two cities, but the same information on both pages, you could either merge the pages into one page about both cities or you could expand each page to contain unique content about each city.

http://support.google.com/webmasters/bin/answer.py?hl=en&answer=66359

I would get rid of it. I would expect that the links pointing at those sites are seen as part of a the same wheel. tiding up the sites will not fix the problem if that is the case.

I suggest to role your own.

My philosophy these days is make sure your site crawls perfectly, use html5 and schema.org to tell the search engine what your site is about. Keep code clean from any script or css, no redirects or noindexing.

It is hard to gain the system by links these days, good content described semantically and clearly to the search engine seems to work well.

It is hard to describe your content using html5 and schema.org using CMS.

I crawled the first 1,000 links on your site and did not come across any 500 errors, I did find you have a huge amount f un-necessary redirects, also a lot of pages with more then one canonical tag in them.

I would suggest that the 500 errors are either on pages I did not crawl or are intermittent, if intermittent then it is probably the server

I had a quick look, your site crawls ok, but I did find 65 broken links, most pointing to http://www.webhostinghero.com/tools/' These are all wasting PR.

I also saw that you no indexed a lot of pages, but you marked them follow, that is good

A few titles and descriptions too long.  and few other problems in small numbers.

For a wordpress site you have done a good job, most wordpress sites I have crawled are a mess.

Moving to wordpress is not a good move, CMS is good for people who need to get a site up quick and don't have the skills or the money to get a be-spoke website, but it is riddled with crawling problems.

I assume if you moved to WP your urls have changed? if so you can expect to lose ranking.

I would go back to what you had before for a start, then have a rethink.

Yes it may, but I thing you still need to beef up the unique content rather then just hide the duplicate content. may be explain to them, that a one page may rank well in all these towns, but Google  is going to look at these 1,000 page s and say we are only going to rank one of them and not very well.

You wont waste any link juice it will come back to you, these are internal links with links back to the home page, linking to these pages and back to the home page will increase the PR on the home page

Please read this page http://thatsit.com.au/seo/tutorials/a-simple-explanation-of-pagerank

the have a play on this calculator http://www.webworkshop.net/pagerank_calculator.php

Maybe this should all be on one page that may rank well, turn a problem into a win

I would say yes it does, simply because displaying outliers on the map is difficult.