1998 Pontiac Transport overheating issues

I have a 98 Transport, 150,000 + miles, 3.4 eng. The problem is intermiittent. I can get in it cold, drive it 2 blocks and the gauge reads in the red and the light comes on and at other times I can drive it for miles and it's fine. I can smell coolant everytime I run the car but it's not losing any. The car is running very rough and the service engine light keeping coming on and off. Sometimes the coolant in the overflow is steaming and sometimes it is lukewarm. I have flushed the radiator and made sure that there is no air in it, replaced the thermostat and the heat control sensor. There is no white smoke in the exhaust and no white paste at the oil fill spout. The oil is clean and doesn't show any sign of coolant in it. The water pump is not leaking or making noise. Last winter the heater stopped blowing warm air but the other problems did not show up until recently. We did have a mechanic change the three back spark plugs on the firewall side, don't know if it could be connected, but never had the overheating issue until shortly after this was done. Just thought maybe there could be something back there that could have gotten knocked loose. Long shot I know but I am stumped at this point.