I would think that if the transmission is slipping,
the engine would still be revving while in forward,
while the power would not get properly transferred
to the shaft. In my case, the shaft spins but something
seems to be blocking (or braking) the whole mechanism
such that the load is too much for the engine to
allow it to revv up.
I do agree that a blown head gasket would make more
sense, so I'll have to make some compression measurements.
Thanks a lot for your input.
BTW, I have read somewhere that a bad coil could also
result in a loss of power while under load. Any comment
on this?