Ford Tough

Hello there! I am a new member to this forum. I am excited to be part of this forum. My first vehicle was a Ford 26 years ago and have been driving Fords ever since. I currently own a '97 and '06 Ford F-150 and a '99 Ford Escort ZX2 Hot.

Question; I replaced my '97's radiator, upper and lower holes. I have filled the reservoir to the "Fill Here When Cold" mark. After driving the truck for awhile, there is antifreeze on the top of the reservoir's cap (over flow ). Also the upper holes is rock hard with pressure. After it cools down, the antifreeze level in the reservoir is at a much lower level and the upper holes is soft. I have refilled the antifreeze level, just for the same thing to happen again after driving the truck on the freeway.

Also my temperature gauge went from warm to hot quickly, then back to normal. Do I have air in the system or is the thermostat getting stuck? How the I resolve this problem?