Cadillac Repair: coolant hot, engine temp, coolant

Question
when driving my 1993 sts 4.6  and a temp of 203 to 208f it sometimes will jump to 235 or 237 and back to 203 within a half of a second and goes into limp mode and on board says [coolant hot] but I dont think car is overheating no loss of coolant and temp does go up ?have you ever seen this before any advice ? thanks mike

Answer
I would replace the thermostat first and if you still have this problem replace the METAL ENGINE TEMP SWITCH which is located on the backside of the engine near the firewall about half way down the engine block and has a green and black wire attached to it.