I had a problem with the forward manifold pressure gauge in my Pitts
that was finally resolved by making the hardline that attaches to the
gauge longer so that it could be looped several inches higher than the
gauge before being routed back to the gauge. My problem was due to
fuel in the line leaving deposits on the inlet to the gauge that would
then restrict the pressure on the gauge and make it read low. Your
problem sounds somewhat different, but if you find you are clearing
the problem by blowing fuel out of the line, rerouting the line so
that the fuel (if that is the problem) drains back to the cylinder
might help.

Dave Swartz
Fort Lauderdale, Florida
http://www.FlightFantastic.US