Losing power

[Oem1952]

I could use some advice on my 1982 non turbo . It sat for a couple of years and I'm trying to revive it . I've replaced both belts and set the valves, now with the injection pump set at .80 the engine starts right up,runs smoothly with very little smoke . The problem is it won't climb a hill . It looses power and the temp gauge rises. Twice now on my drive home from work this has happened , and at exactly 5miles after climbing a hill it looses power and stalls I pop the clutch and it starts right up again . Driving around town on level ground at under 40 mph it's fine , peppy and doesn't overheat . Does this sound like a valve/timing issue or am I sucking air ? Any suggestions would be appreciated .