1992 Ford E-Series Van blowing radiator end caps

my van doesn't overheat at all , the water pump seems to be in good shape replaced it 4 years ago, replaced the fan clutch this year, the thermostat was replaced 2 years ago, the radiator cap has also been replaced 2 years ago. the vehicle is well maintained the motor runs fine, no smoking exhaust at all. its seems to pop the radiator end caps once a year. the radiators are lifetime replacements, but it sucks to change them each year. i think the water pump could be building too much pressure but the radiator cap should let it off. so i'm thinking bad parts, it is an aluminum radiator with plastic caps. so what do you think could be the problem????