Radiator leak and wild temp gage?

[sprucegoose]

I'm having trouble right now with my cooling system in my 98 Takoe, and wonder if anyone has any good suggestions. My wife called me the other day to tell me she smelled coolant while driving it, and when I looked underneath the drivers side front and wheel-well were soaked! Coolant overflow reservoir was almost empty. At first I thought it was maybe the drain petcock leaking down there. After cleaning it all off and running it, I could not see the leak anywhere! Checked the hose connections, etc. as I had replaced the radiator a month or so back with a warranty unit. (last one was leaking from the side tanks after only 6 months.)

Now yesterday I was test driving it and noticed that the temp gage spikes to about 215 F or so before the thermostat finally opens the first time. Used to be about 200 or so. The wierd thing is that it cycles between 200 and 160, going up and down virtually every mile or so as you are travelling down the highway at 60 mph! It never did that before. Usually would level of at 180 of so, fluctuating maybe 10-15 degrees total one it heated up. At idle it doesn't seem to get this warm, just stays about 170 or so on the gage. What could be going wrong here?

One thought is to change the thermostat, cheap fix which I probably will try first. But that does seem to be functioning, and doubt that it would cure this... I just see this coolant as getting hot very quicly in the motor and am wondering if it's got a head gasket going? This is a GM rplacement motor and has about 90k mi. on it.

I also thought the leak may have been caused by pressure build up. When my side tanks went last time, I did replace the radiator cap also so that should be fine. I don't see any bubbling in the overflow tank at all that could indicate a head gasket problem. Any ideas??