1993 2.5L Plymouth Acclaim eratic dash temperature guage

I had a new rad (1 week old) put in and 2 new different rad caps. 2 new thermostats, new coolant, 8 month old water pump installed. After driving driving a distance the temp. Gauge starts fluctuating from to 1/4 to just below 1/8 mark on the cold side range. When I open up the hood and leave the car running, the rad cap is warm and it seems that there is no pressure build-up in the radiator because I am able to open it without the coolant squirting out. Some people tell me that it is a head gasket problem but I am not sure. Because I there is no external leak and no white, sweet smelling exhaust coming out of the muffler. Does anyone have any ideas? Any input would be very much appreciated.