Possible head gasket? I'm stumped.
 

Last night I was driving about 40 and the temp gauge in my 01 started moving towards 260. I let off the throttle and coasted to a pull off. The temp started to drop back to normal as soon as I put the truck in park. I popped the hood, motor wasn't even hot. There were no leaks that I could see, but the truck held 3/4 gallon of antifreeze, then drove normally the rest I'd the way home. This morning, I checked the oil: no creamy milky mixture. I opened the radiator cap and turned the truck on: no bubble or shooting. There is also no white smoke out of my exhaust. I plan to have it pressure tested to make sure there isn't a small leak somewhere, but if that test shows no leaks.. what's the next step?

Get a hydrocarbon test on the coolant. That will tell you if there's a head gasket leak that's seeping into the cooling system. I was plagued with that for about 5 years before the test confirmed it.

I've had a few thermostats going bad, do this same thing, they start to stick, and then wont open untill it gets excessivly hot then breaks loose, but a low coolant level can do the the exact same thing, a good time to check for a leak is a warm up in the morning, with everything cool in the morning just start it and watch for drips as it warms up, most small coolant leaks (mainly hose leaks) will only leak when the system is cold and will cure themselves as temperature rises and thats why they are never noticed

So you're leaning toward a tstat?

well.... if your coolant level was low, and from what i get, u added 3/4 of a gallon to the system? if so i would start with just keeping an eye on it and see if it happens again, the temp spike would probably be caused by the low coolant level, then start trying to find the source of the missing coolant