95 jimmy heat problem HELP

have a 95 jimmy temp runs fine on the highway i put the heat on and the air is warm when i drive in the city and stop at a light the gauge starts rising passed 210 and the air turns cold(coming out of the vent)when i drive again the gauge drops and the air turns warm again.cloged system i own the car for 4 yrs now never flushed it.someone please help :frown:

This sounds like a partilly clogged cooling system. Take a look first and make sure the system is full and is not low on coolant from a leak etc. Take the cap off the rad and take a look in there to see if its low. Also, take a look in the rad and see what it looks like. Does it have a lot of build-up/rust deposits? If the coolant hasn't been changed for at least 4 years, it would be wise to do a complete and through flush and refill, especially if your using dexcool.