Okay, based on a couple of responses, it sounds likely that the prop is
probably just too much for the engine (especially at altitude).

Any recommendations about where I can buy good quality props cheaply? I'll
hit Google and e-bay, obviously, but if there are any particularly good
on-line shops, I'd appreciate pointers. OTOH, since props are bulky and
heavy, the shipping costs may be heavy.

I also have to figure out the exact specs of the prop I need. I'll look at
my Merc manual and the props I have, but again, any pointers would be
greatly appreciated.

Thanks again,

Shawn.