Engine temp is normal at all rpm's, I so I don't suspect a thermostat prob.. thinking blocked heater core? No coolant leaks anywhere..

 

And if you open the radiator cap -WHEN COLD - the coolant is all the way to top?

 

Yeah, coolant levels are all good, in the rad & the reservoir tank.
Just this week out of the clear blue the heat is acting up.
If run at high rpm's the heat works great, but when run at lower r's it cools right off.

 

If it goes from cold to hot in a couple seconds, that is not a good sign. If it's hot as it used to be when it is hot, your core is not plugged.

Does it stay hot while accelerating to highway and when passing? If it starts cooling down then, perhaps vacuum actuator problem.

Try this:

Drive the truck around town, till the temp hand shows normal temp for a few minutes. Then shut it off and watch the reservoir for bubbles.

 

The engine temp is stable, at any running speed whether it be fast or slow.

If I drive with rpms over 2k rpms, the blower heat is great. If I upshift so the rpms drop to below 2k, then the heat from the blower motor cools down in a matter of seconds.

 

All's good- low radiator level, thought it was full, but not (my bad!). Problem solved, replaced the rad cap & thermostat anyway. Thanks for the advice!!

 

Check for coolant leaks!! It shouldnt just go low like that!!