what is causing my 94 mazda mx3 to not accelerate going up hill

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my 1994 mazda mx3 5 speed manual will not accelerate past 23mph on an incline. This began after I gunned it trying to get it to start on a very hot day. It drove fine for about 5 minutes then, would not accelerate past 20 mph, and would not gain any speed at all, it would rock violently when trying to accelerate it. Left it alone for a week, it ran fine around my area where speeds don't exceed 25-30 mph. Then when I took it up hill onto a bridge it would not accelerate over 23mph on the incline, righted itself at the summit. The catalytic converter looks good, the air filter is fine, the plugs and points are fairly new, it is a well maintained auto. What could be causing this problem