RE: Thunder Cat 31
Dan.revo
It sounds like you have more of a tuning problem. Even with cavitation it should get up and go within a few seconds. Check your bottom end setting, sounds a bit rich.
Drmeff
Move that feul tank. I found the hull to have a slight hook which forces the nose down added with having the feul tank in front of the CG it mskes the boat prone to oversteer. I dont know what radio you have but you should also check your amount of steering through.