Well with all the stuff you've checked i would say chassis flex also.

You say you have a Hyper 8? Have you installed the stronger center diff mount that the rear chassis brace keys into???


http://www.nitrohouse.com/options_hyper_8.htm

That brace was partly for problems in the beginning with extra chassis flex. I think this is standard on the Hyper 8.5's but it was an option on the 8's.

Also is your kit new (to you) or have you had it a while??

There were also issues in the beginning of the Hyper 8 run in that the center spur was out of round. If you have one of those spurs you could be setting mesh loose on on part and tight on another part as it spins.