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I'm currently starting to optimize about 30 area specific pages for a property maintenance company and was wondering, what's the best way to link build multiple pages at once? Should I link build for all of the 30 the pages at once, or maybe focus on 5 individual pages - get them ranking well, then focus on the next 5 and so on?

The website for my London based plumbing company has thousands of specifically tailored pages for the various services we provide to all the areas in London. It equates to approximately 6000 pages in total. When google has all these pages indexed, we tend to get a fair bit of traffic - as they cater pretty well for long tail searches. However, every once in a while Google will drop the vast majority of our indexed pages from SERPs for a few days or weeks at a time - for example at the moment Google is only indexing 613 whereas last week it was back at the normal ~6000. Why does this happen? We of course lose a lot of organic traffic when these pages don't displayed - what are we doing wrong? Website: www.pgs-plumbers.co.uk