[Richsgt]

Definitely try using a wheelie bar, just a simple spring loaded device you could even trigger via a servo at the start of the match to support the robot in the rear of hte machine, it appears from the picture to have most of the weight in the rear. Since your front wheels aren't powered, or so i take it they aren't going to counter or help ont he ramp at all.
What's probably happening is like when a motorcycle spins out, it 'pops' up the front end of the bike and in the case of the robot makes it extremely unstable. Best solution is to figure out the CG of the machine and then figure the length of the wheelie bar needed in the back to counterbalance it(and then add a few inches because there's always something thats forgotten).
If you have more time, and a little weight, try moving the wheels positioned in the front up a bit, or making the front wheels wider(say 2"), put some tread on them and smaller so the robot 'leans' forward a little to try and counterbalance whats happening. As for the rear wheel, move it up a couple of inches or figure a way to shift the CG overall closer to the front of the machine instead of the back, but again all depends on fix it time, and the weight u have to play with. Hope this helps