My '82 2.6 liter Ram 50, close to 190,000 miles , has been overheating/losing power when hot, such that I turn on the heater full blast, open the windows, and slow down from about 65 to 40 or even lower, when climbing to 6000 ft east of San Diego, in the late after noon in late spring or once in June, for recreations , abut once a month.

When starting cold , seems that at least one cylinder is missing, (vibration) for 3 or 4 seconds, then smooths out with some more throttle.

Muffler (?) rattle in reverse. Smog inspection coming up in August. Passed OK two years ago, with a new lower catalytic converter. I put replacement exhaust manifold on it about 3 or 4 years ago. Old one was cracked. Also cleaned off deposits on the upper cat ( or maybe I just removed the upper cat core).

The head was resurfaced in Ensenada about 4 years ago. But later, the crank bearings scuffed when I overrevved for several minutes , via absentmindedly failing to go from second to drive on a long downhill.
an obvious overheat. Crank and rod bearings fixed. Maybe I warped the head , again.

Before that, mechanic in Ensenada said that valve clearances were correct. I have not set or checked the valve clearances myself for years.
Maybe head bolts should have been retorqued after 1,000 miles , after the head resurfacing? But a bicycle collision complicated my life, and I did not drive for at least two years.

Oil is full, coolant full, radiator should be healthy, with no rust or oil noted in coolant, which was regularly changed in years past. Have not checked the fan clutch