Hi I have a 1997 Monte Carlo Z34 v6 3.4 Rough idle, Idle revs to 2000rpm repetitively, car has lost most acceleration and sometimes the car dies. The car also heats to 260 after about 4 minutes of driving. Error Codes P0121 P1655 P0300. No leaking of coolant, just changed the thermostat.

 

you might check the coolant temp sensor connection (located by or on the thermostat housing). make sure the connection is good, and make sure the sensor isn't physically broken.