Loss of power

Looking for a suggestion from the list
Car started fine, ran for an hour and lost power. Would run
and rev but no torque.  Pull out manual choke (hot engine)
and now there's enough power to get it home. Car recently
had head gasket replaced, there doesn't seem to be any oil,
water, or steam in the wrong places. Before this, car ran
strong.  (1500 engine)
Any ideas?
George Procyshyn