Chevrolet Repair: Cooling system?, degree thermostat, radiator cap

Question
I have a 1994 K2500 Chev. , 350, automatic transmission.  Recently the engine began running hot and cold. I installed a Hypertech computer chip and a 160 degree thermostat last winter. I don't think the chip has anything to do with the problem, but thought I should mention it. It has run fine up until recently. Running around town, it never makes it above 130 degrees. On the highway it will run hot uphill (as high as 210) and cold downhill (as low as 120).  I have replaced the thermostat twice, the fan clutch, and the radiator cap.  There are no apparent cooling system leaks and the fluid level is full. I change the coolant every other year. There is no oil or buildup apparent in the radiator.
I was thinking it could be a sending unit problem but don't know. I am completely confused at this point. Any help would be appreciated.
Thanks

Answer
Hi Bob,
An overactive sending unit may be what is causing that, but 210 is by no means HOT.
Make sure the coolant is full inside the radiator. An air pocket trapped in the system could cause some funny readings also.
There are charts for resistance values of senders at different temperatures, in order to test them.

Van