I have a 1999 town country I have poor heat and when I got into the express way the temperature drops down to nothing heat some body can help me please?

First choice would be a new thermostat. Could also be a clogged heater core.

Reverse flush the heater core and refill with a 50/50 mixture of antifreeze and distilled water. It would be best to do the whole system. If the problem persists, check out the blend door's operation.