Engine repeatedly overheats & cools down, over & over again while driving on 1990 Chevrolet 2500 Pickup

A few days ago I stopped at a store & when I started to drive again, I noticed that my engine was overheating. Just as I noticed it & was getting ready to pull off the road to stop I noticed the temp. was rapidly decreasing, so I continued to drive. This happened a couple times, so I thought it was only happening when I slowed down or stopped & started driving again, but then the next day I noticed it doing the same thing while continually driving around 55 MPH. What would cause it to rapidly over-heat & then rapidly cool down while driving a steady speed, over & over again? Would it more than likely be the water pump or possibly be something more serious, like a cracked head?
Btw, I havent noticed any fluid leaking anywhere either.

2 answers
Had same problem with my 89 2500. Turned out to be a head gasket. Needs to be checked quick before antifreeze wipes out piston rings.
FIrst and foremost, check the radiator fluid level. A low level will cause that exact problem