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There was quite an interesting thread about those:
I'm experiencing this too but the ads seem to go back online the next day. Ads with less than 1000 impressions with CTR numbers greater than 2%. It's annoying but I don't loose sleep over it. Maybe there was a lag in registering the impressions and the CTR wasn't really that high. Maybe it's a glitch.
* This keyword has been disabled because its clickthrough rate (CTR) for the last 1,000 ad impressions it generated on Google falls below the minimum required CTR.
Did you consider that they rate it on the last 1000 impressions and not on the running total of the CTR? Example, I had ads that were over 1.3% CTR but slowed down during the night. The total CTR showed .9% CTR but the last 1000 impressions only had a few clickthrus which caused the ad to disable.
It does seem strange that you're getting disabled with a reported CTR of 1.5% though, as that would imply that you're getting many times the CTR on syndication partners that you are on Google.com. One possibility is that your number of impressions so far is quite low (below 1000 or 2000) and so chance effects like this could be present - is that the case?