You most likely have air pockets in the cooling system, probably in the engine block, near the temp sender...mine did the same crap after i changed my t-stat, i tried a new cap also, didnt fix the issue...
try this procedure...you need access to a steep incline, so you can point the nose of the truck upwards...like park it in a ditch line if available.
This needs to be done when the truck is completely cooled off...like in the morning.
park it on steep incline, nose of truck in air...have jug of coolant ready to pour in..
remove rad cap...
start truck...(its good to have a helper to run the engine at about 2000-2500 rpm during this procedure)
keep an eye on the coolant level in radiator, continually adding coolant as the truck "burps" out the air pocket/s in the system, you should see them bubbling out of the radiator as the truck is warming up....
at some point, the truck will stops burping out the air pockets and start pushing a continuous flow of coolant out of the rad....this method of removing the air out of the system will waste some coolant, so don't freak out when it pushes coolant out of the radiator, it's part of the process of getting the air pocket/s out of the system...
good luck...