2001 Chevy S-10 surging in overdrive.

I have a 2001 s-10, and for the last 1 or so when the engine warms up, and the oil pressure levels off while driving at highway speeds(45-55) starts surging. It only does it in overdrive, when you accelerate or let off the throttle it stops. You can literally watch all this happening on the oil pressure gauge, when this happens the gauge jumps forward as well. I was told that it was the transmission and I was already having transmission troubles anyway. So I had the transmission rebuilt, which fixed all my problems but this one. The check engine light is not on, and I even tried a tester in case it had burn out( of course no codes). My father has told me that he thinks its a misfire, but ive tried new plugs and wires. So my question is could the coil pack be causing this issue. And does anyone else have any ideas? Any help would be appreciated. Thanks