my 96 SSE had this all the time. Changed the sensor. Still did it. After a real good oil system clean took care of the problem. I don't remember what the guy did but, man, did it help.
P.*. your pressure never really actually will get to 120 psi. It'* like the fuel gauges on just about every GM I've owned except the two bonnies. Its a kind of a sort of maybe a target more than an accurate read. Both my GP'* had terrible fuel gauges and the 91 GP never read the correct water temp.
1996 SSE Silver Toupe
2004 GXP Crimson
1992 Nissan Sentra
(Goners) 91 Grand Am LE; 91 Grand Prix SE2; 95 Grand Prix "Special" Edition