I am having trouble with engine temp indication. Once the truck gets to normal operating temp the guage starts to slowly rise to high normal then after a few seconds it drops rapidly to low normal. It stays a low normal for about 30 seconds then starts the whole cycle over again. It has plenty of coolant, just changed the temp sending unit out, not sure if this is a thermostat going bad or what. Any suggestions?

1996 F150 XLT, 5.0 V8

 

That's just your thermostat opening and closing. I've had a few do that. If it bothers you just put a new one in. You'll be good to go. It won't hurt anything the way it is.