[cancobra]

I have searched the forum for a similar issue to what I am having but have not come across anything yet.
I have a stock 3.2 in a 89 911 which has just had a major reconditioning which included externally new coil, fuel pump, distributor cap, wires, sensors, battery, fuel relay and also extensive internal mechanicals with the engine completely apart, essentially every thing replaced that needed to be replaced except pistons. Basically a rebuild.
I have 2 problems I just can't seem to solve. When driving the car at a constant rpm it seems to be jerking meaning it feels like someone tugging at the rear of the car lightly all the time. It is noticeable at all everyday rpm driving, for example 2000 to 4000. The car accelerates strong and without any problem.
The second problems occurs after about 10 minutes driving the car if I am in 4th gear and cruising the car around 3100 rpms it will give a noticeable jerk and the tachometer will drop to 2000 then come back up. I have attached a volt gauge in various locations to try and find out where this electrical problem is occurring. I have wired it into the fuel relay switch and at the fuel pump etc.
I have starting to think this may be a problem with the "computer" or "brain" as it is sometimes called. I wanted to avoid replacing it as I am told it does not fail very often.
Has anyone else experienced this kind of problem.