1991 and hesitation and some strange vibration

Hi there, I have a 1991 PA and I'm so confused what's going on with it.
First of all, my car start every time as it should be, fine and smoothly.
Idle is great and smooth also. It runs as it should be as many ways.
When I go to the highway and ran using cruise about 35-45 Mph, most of time it's ran smoothly and nice.
But, when it raise slightly speed using cruise (or my self manually) my car hesitate and jerk just when it moves a speed. So I mean that if I use gas pedal smoothly my car hesitate but if I use pedal hevily it rans nice and I think that I have a all power on it.
Some times,. not avery time my Oil pressure meter coming crazy, same time when I slighty push pedal, oil pressure meter raise to the end. It happens only on that time, usually in the city I can't notice that because I use pedal on heavily.

I allready change plugs and wires, oils and filters ...
I can't see any broken connection or such things under the hood.

I don't have a idea what's wrong with it ???

Regards,

Risto (from Finland)