You could have a couple of problems. The oil pressure indication problem may indeed be the sensor (sending unit). You can get a replacement sensor at most auto parts stores and changing it is a snap. If you remove the electrical connector from the sending unit and there is oil in the connector, then it is definitely bad...what happens is the oil leaks through the sending unit from the oil side to the electrical side and causes erratic readings on the oil pressure gauge.
The other issue with the rough running should be troubleshot as described already...
