It could be multiple issues but the cheapest place to start is with an alignment. I would suggest going with -.2 to -.4 degrees rear camber and zero toe; for the front I would go with -.5 to -.8 degrees of camber and -1/16 toe in. This is a great street/strip alignment.
With the HP you're making I would also strongly suggest going with a good sub-frame bushing replacement. We offer the Pedders bushings as well as the Pfadt mounts. They are both excellent products and they will keep the sub-frame where is is supposed to be.
I would also suggest coilovers with that kind of power; the increased spring rate would make it feel much more planted. Please feel free to call, email or PM me anytime with questions. I'm always happy to answer question and find the right solutions to obtain your goals.
Kind regards,
Jordan Priestley
888.308.6007