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Seems to be getting more and more common, I've especially noticed it over the past few weeks. Remember that browser plug-ins are still using cookies and localisation information based on previous searches and data. I had it happened to me last week with a keyword, the first search came up at position 33 I clicked on it to check the link, with a second search a few hours later it came up at position 4, quite a big jump! I guess that aircon installation is very localised compared to say an e-commerce store, so I guess Google will be working very hard to provide the best result for the customer weighing locally relevant results heavier. Your safest thing to do is to use a good online system and make sure you set it for the relevant country.  Clearing the cache each time prior to doing a search should also help.

Thanks so much for the responses, they are making a lot of sense. I am going to look further into what you have shown me and try to get better DA links. Regards Joel