Way back in the mid 80s Chrysler suggested you replace the thermostat and gasket every 2 years. The reason was because the old ones would deteriorate and would open before reaching the desired temp.

Low heat out put, first thing to check is the coolant level and freeze protection. The second thing to do is replace the thermostat and gasket. Back flushing the cooling system every 4-6 years is a great idea. After that, you troubleshoot the condition of the heater core. Gene