97 Astro 2wd blower stop/start engine load

Newby
Problem just started this year's vacation after owning since new, summer mountain highway driving - My interior front fan blower stops under very heavy engine load (climbing a hill on highway, hot outside), then restarts immediately when engine load off at crest of hill. Behaves the same if A/C on or off. Seems weird as no a/c interlock apparent, is the blower switch control via vacuum by any chance?
Had a previous problem where vents would not change from hi-lo-hi/lo-defrost, fixed by replacing section of over-heated and brittle/broken vacuum line over bell housing into the dashboard control switch.
Any ideas would be appreciated.

Does the fan motor stop or does the air stop blowing out the dash vents? If it stops blowing out the dash vents it is a vacuum problem...probably there is a check valve in the line that has gone bad.

Darn - that's a good question. I am pretty sure that the blower actually stopped (but lots of road & engine noise at 70mph). Blower was set to highest speed, did not hear blower at all during failure mode.
My recall is that on previous vacuum vent control failure, venting failed to the "defrost" position, fan continued to run all speeds. Should have still heard the fan if only vent control failed.... may have to take another highway holiday!