Chevrolet Repair: 96 Lumina overheating??, chevy lumina, temp gauge

Question
my 96 4 dor chevy Lumina has just recently started overheating according to the temp gauge yet when I check it it doesn't give off extreme heat like it is hot and water temp is not high but there is ussally no pressure on hoses and this morning when it said it was hot and I stopped to check it both the upper and lower hoses were sucked in as if the motor was in a vacuum someone said it might be the cap I was thinking T-Stat please see if you can point in right direction

Answer
hmm check the cap and if the rubber is shot replace and check the thermostat and i would replace it too. but if both hoses are sucked in i would check the fluid level and then check for a blockage or check the water pump, it seems like its working s i am going witha  blockagae of some sort.

let me know what happens.