MMM (and MMP) cogging/hesitation when warm?

[bdebde]

I have been fighting this problem for a while now, and finally got it fixed this weekend.

I started out with an older MMP and new Neu 1512 in my 1/8th buggy. After driving a few rounds with it I had some intermittent cogging/hesitation problems. I changed both the controller and the motor, because the connectors were different on original setup and new setup. The new setup was a new MMM and a used 1509. After several rounds I had the same problem again! WTF?
I figured out that I could run 5 or 6 minute qualifiers without problems... the problem came about in the 10 minute mains towards the end of the main. I figured it either had to be heat related or voltage related (voltage sag at end of pack). I was not running packs beyond 70% discharge, but the were older 30C packs. I replace them with new, slightly higher capacity, 65C batteries... still no change. I even replaced the receiver to eliminate any other electrical issues, thinking that both motors or speed controls could not have had the same problem.
I put the Neu 1512 from the original setup into another rig... it worked fine, so the MMP must have been the culprit in that setup. I then swapped out the Neu 1509 for a CC1512 in the latest setup... still the same problem. That pointed to the brand new MMM being the culprit in that setup. I had another new MMM but was unwilling to risk it since this was a big race this weekend, and took a known working MMM out of another rig and all was well.
Only problem is, the bad MMM was a new replacement from castle, but had been in the toolbox a while. I hope someone from castle reads this (otherwise I will have to call them) and can help me out. I don't want to have to fork over more money for a replacement for the replacement that never worked in the first place. I should probably also check the other new replacement, as they were received at the same time.