What is the problem when a car lurches instead of automatically shifting? I am talking about the '86 El Camino. When I am trying to keep a constant 35 mph, it sometimes 'lurches' instead of shifting to a higher or lower gear. It's almost like it gets stuck at 35 mph.

I try not to keep it 35 for that reason, but sometimes I have to slow down to the speed limit. My car just does not like to go 35 - it acts funny at that speed.

Any guesses?