450 SE light throttle problem

[Chris 450SE]

Can anyone help me with a problem? I have a 1989 450 SE with a light throttle problem. If I accelerate hard it goes like S**t of a shovel, but if I get stuck behind anyone doing say between 30 and 60 it stutters something bad. The car has a full history and I have changed the coil,throttle pot,plugs, leads, rotor arm and distributor cap. Sometimes I will be driving with the stutter and suddenly the car accelerates and the engine sounds happier and for the rest of the day I can drive at any speed on a light throttle without problem. Somedays it will never run right and my idle speed is extremely low, if I put full lock on the steering the engine will cut out. Please help me as it will soon be summer and I want to cruise without problem.