Pushing off-power and then spinning on is a classic "your tires are too hard/bad" scenario.
thirdplace actually did extensive testing with the RC4less tires for roadcourse and I ran some as well albeit, I gave them up quickly - believe me when I say that they perform as being much harder than what they say they are. Just for example, an RC4less pink front gave my otherwise balanced car on TRC Pink fronts a push I couldn't get to be that bad on any other brand Purple... Their foam is just “different” – great for oval though…
What are you running for side shock oil/springs?
You setup sounds mechanically "grippy" - I can't see anything on the suspension itself that jumps out at me unless you come back and tell me you have 40wt in the side shocks...
I would try different tires and different sauce - we actually switched to sunscreen when trying to loosen a car up. Jaco, TRC or now, GRP, Green fronts with White rears - if that doesn't get you some bite, then start looking for other things wrong.
A classic mystery issue with pan cars is the lower rear pod plate - if it's bent/warped/weakened - even just barely visibly, the whole car goes haywire just as you describe.