Okay, that doesn't sound like an air bubble or a leak. Probably not a t-stat either, but I'd still change it as "routine maintenance"..........who knows, maybe you'll get lucky. This sounds more like a problem with the radiator or fan clutch. Make sure your fan shroud is installed correctly and the lower piece is there. While you're underneath, check the front of the radiator to make sure it's clear of debris. Radiators can also get plugged up on the inside and flushing doesn't always help. Does your fan clutch roar when it's cold? A working fan clutch will roar for the 1st 3 or 4 revs when cold, but will then quiet down when it starts slipping. Sometimes they will pass the "cold roar test" but have a stuck thermostat. There is a bimetal coil on the front of the fan clutch and it's job is to activate the fan clutch when it gets hot. Sometimes that fails and/or the shaft it turns gets stuck. If you take the shroud off you can test it like I show in this thread:
http://www.toyotavantech.com/forum/s...the-clutch-fan . Have fun
. Tim
PS: If it's starting to overheat, you can turn on the heat inside the van to help your engine dissipate heat. If that keeps it under control that's a good indication it's a plugged radiator or an air-flow problem. Could still be a t'stat though. Keep in mind that the engine thermostat does not affect flow through the heater cores (heater system bypasses the engine thermostat).