Ads not 'sticking' (losing targeting)

On infrequently visited pages

[ari11210]

I have been running adsense basically since the beginning in 2003. I get mostly targetted ads on my site's pages. But, I have one page where you pick a city in the U.S. and submit a form to get a content page with information about that city. So, I would prefer that the ads for that page are targetted for that particular city, and they are after the page is spidered by the media bot. But, what happens is that certain cities are not very popular and lose their targetted ads after a while of not being visited, and go back to generic theme-based on ads on the sites general theme. While, the popular cities almost always have ads targetted to the city.
Does anyone know a way to stop the ads from forgetting their targetting? What I was doing for a while was visiting every city's page once a week to keep the ads up, but beside being cumbersome is probably against the TOS, so I stopped that. I am even using the 'google_ad_section_start' tags.
Basically, does anyone know what is going on? Why are my ads losing their targetting if not being visited frequently? If I dont solve this, I might have to find other ads to put on these pages. This is not an inventory issue, as the ads are targetted initially, but then lose it.

Thanks
Aaron

[JohnKelly]

If the city page is being generated by a form POST, then perhaps the adsense bot can't "see" the page.

Have you tried caching the page as a static file to see if that works.

Or have a separate static HTML page with generic city info AdSense on it that is included in the form POST output via SSI or PHP.

[ari11210]

Thanks for your response.
I use a GET for that reason, it also has the added benefit that the URL has the city name in it then which can help the targetting (for example /mypage.php?city=New York)
Making static pages might work, but that would mean creating a static page for each city (over a hundred), which I am hoping isnt needed. I now have one PHP that just takes the city name and generates the content. I know google could see that page since it does get the content targetted for a while, but eventually forgets it if it isnt visited often. Any other ideas?