Hey guys im having two issues. first one is the oil pressure gauge was reading 40 psi the whole time i had it. Today all of a sudden it started going from 40 to 80 at random times, what can cause that? Second issue is when driving at higher speed 55 and up seems when it hits overdrive it like vibrates then when i hit the gas a bit it goes away and then it comes back.

 

On my 89 S15 Jimmy I was having problems with excessively high oil pressure readings. Oil would blow out of the dipstick tube if I did not have the dipstick fully seated. The PCV valve would blow out of the valve cover. It took me a long time to figure out the problem. I decided to rebuild by TBI and during the process I found that the pcv port in the TBI was plugged with carbon build up. Once the TBI was cleaned and reassembled in installed my high oil pressure was back to normal. As a test you could remove the tube from the PCV valve and let it vent to atmosphere and at the same time make sure you cap the port at the TBI to avoid a vacuum leak. As far as the trasmission is concerned I would check the fluid level first and look to see if it is burnt at all. Any chance that the drive shaft is out of balance? What is the condition of the transmount? How are your motor mounts?