OK, so my 95 taco with 116k is overheating...I swapped the thermostat last night, but it did not fix the problem.

The truck runs fine, but after about 5 miles on the road the thermostat goes way up, then back down...then bak up and down...and so on.

It never goes into the red, but it gets really close...

any ideas on what the problem might be?