another cooling issue

hey all, i have an 87 that has a strange issue... it never gets to running temp and hangs below 100 degrees except when i sit at an idle in 5 o clock traffic and then it over heats and that stupid lil plastic bottle starts hissing and shooting out coolant everywhere. so far i have replaced the water pump, thermostat, checked to make sure the fans are working (yes they both are) and got a new cap for the bottle... any ideas?
thanks,
jmeti

replace your radiator. and your overflow cap. radiator is 100$ from radiator.com, usually delivered in 2 days.
when the cap goes bad it allows for pressure to get outta your pressurized system, causing probs. so get a new one.