instrument gauges

i have a 94 mustang 3.8. when i first start the gauges read normal. after a while the voltge gauge moves to the low side and the oil gauge moves all the way up where it cant go any further. the temp gauge works its way up also. when i turn on the lights and ac and then turn them off the gauges move down a little. i have a new battery and had the alternator bench tested 3 times and it passed all 3 times. on occassion the car will buck strongly for a second or two(not sure if its related)any ideas. the last scan i had checked out ok. i'm puzzeled. can you help? thanks.