Here's one for the masses:

In the past year had my bottom end rebuilt.  Within the past few months had
both heads/valves redone.  Van runs great around town.  Soon as I head out
on the highway, about 20 miles or so, I begin to lose power, ever so
slightly so that I can't exceed 55 or so on a flat road.  If I hit a hill
and have to downshift, I lose even more speed/power and drop to the low
30's in 3rd.  Mechanic says I have good compression all around (140 or so).
Also:  Starter will not engage if the engine is turned off then attempted
to restart soon after this 20 mile trip.  Will restart with a jump later.
Should I get a new mechanic, or is there a solvable mystery here?

TIA,

mark