I had the same problem when I purchased my previously owned 1994 Lumina APV w/ 114,000 miles. I changed the coolant temperature sensor and did a radiator flush. Not the 10 minute flush but the "cleaning" which requires one to leave the cleaner in the system between 3 to 6 hours of operation. A lot of sludge came out. I believe the radiator was partially plugged. It is worth a try before you spend a lot of money at the mechanic shop. Hope it helps.
The coolant temperature sensor is located at the rear of the engine next to the oxygen sensor, just behind the bracket that holds the ignition coil.