FWIW i have seen several A/C compressor clutches around here in NC (non MB, although it should still be the same concept) do the same thing. You can smack the center of the clutch with a pry bar and they work again for a little while and then quit. Two things come to mind:
1.) The air gap is set on the high end of acceptable parameter and with heat, it expands to beyond specs.
2.) Clutch coil is weak, due to high resistance. With heat comes higher resistance and eventual intermittent failure.

Do you know if there is a signal at the clutch coil (at the plug) after this happens?

__________________
Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi