1992 Chevy Silverado Can't Figure it out

my truck is overheating by 30 or 40 degrees this started happening after i put a new waterpump and resealed the intake manifold. i thought it might be the theromstat so i put a new one on it there are no pressure or physical leaks or cracks on the engine or the radiator, i also put a new upper hose on the radiator. I just can't figure out what the problem is. can you please help