oil gauge bounces

Ok Car was a 4cyl.. now has all this is in it.... was a drag car on nitrous etc.. 5.0 bored .30 over to a 306 T5 5 Speed Manual Transmission GT40P Heads GT40 Upper and Lower Intake 4.10 Gears 8.8 Positrac Rear End BBK 70mm Throttle Body BBK Shorty Headers MSD Street Fire Ignition 140mph Dash...

Howdy! First post here! I have a problem with my niece's 98 Stang. When it gets warm, sometimes the idle will freak out and act like its going to die. Same time the oil "gauge" freaks out and starts bouncing! That happens at lights or parked. After we start driving it, that goes away tho. Once...