1989 Dodge Ram overhearing

Hey,

My van is overheating very fast (within a mile). I just replaced the thermostat and radiator cap along with the hoses. When I did that the problem was fixed and everything was working great for 3-4 days.

Then it started to overheat again one night and the heater started to only blow cold air.

There are no visible external leaks. Both hoses get hot. I don't know what to do next?

Could the thermostat have gone bad that fast? Before the heater blew hot air just fine.

There is plenty of coolant in the reservoir.

Any advice would be much appreciated.

Thanks,
Trace

Parts are extremely cheap plus I get an employees discount at a parts store.