Q: my car is almost dying and at 40mph wont go over 2000rpms sometimes under that.

i have a 2002 gmc envoy and recently i was driving it when all the sudden i hit 40 and my car started to vibrate really badly at 2000rpms while i was accelerating. i got to the top of the hill i was going up and pulled over and checked my oil and gauges and looked under the car and nothing out of the ordinary. I do however have a problem with my oil gauge currently where under 10 mph i loose all pressure in the gauge till i stop then it jumps up and down. i check my oil everyday and it seems fine. When i got home from the fist part of this inquiry it kept dropping below 500rpms and wanting to die so i had to keep revving it to park it. if it's anything super major i would like to know so that i can either A take it to a shop or B part it out and get something new. Mind you i have already put $1500 in sensors into it during summer when my gas tank sounded like it was boiling.

My car has 158000 miles.
My car has an automatic transmission.