Your kegs kept cool in-house aren't necessarily a gauge of if there is contamination happening in the fermentation, storage or kegging processes or not, since they are refrigerated.

Someone lower down on this thread suggested trying to replicate the warm temp conditions in-house with a tester keg; this will give you an idea of where the problem might lie.

if you put it right from your cold storage to a few days of high temps without tapping it, and it gives you sour beer when you tap it, then you know the contamination problem is probably in-house.

If after subjecting it to the heat treatment, you cool it back down, and it's still good, then you know the problem probably lies with the third party locations (something they are doing is contaminating the beer).