A lot of times, a stumbling response to throttle is an ignition problem. I've been fighting a bit of this problem myself and (fingers crossed) seem to be making progress with a new rotor/distributor cap and swapping the coil-to-distributor hot lead with another one from my "parts box".

I'd suggest you look inside the distributor cap to see how worn and burnt the contacts are and see how much oil has leaked into it. Also, it sometimes helps the Regina cars to take the coil apart and clean the internal contacts and all the mating surfaces.

Just a couple of low-cost/no-cost suggestions. Good luck.