This is a bit long winded but I'm looking for qualified opinions as to what
might have happed to an almost total loss of power while in cruise flight in
my Mooney M20F. Here is the story of what happened:

I was flying from Turlock (south of Modesto) to Petaluma (north of San
Francisco) on a Saturday. Weather was strange with a thick mist and fog layer
from about 500 feet to 2000 feet. I was VFR on top and there were no holes
anywhere through the fog/clouds/mist.

Prior to the flight, I sumped the wings with no indication of any water. I
did a normal run up and there were no issues at all. I was running on the
left tank which is what I had run on for the last 1/2 of the flight the night
before. Left tank had about 20 gallons right tank about 30. Take off was no
issue and full power was available with a good sounding engine. I found a
hole in the clouds and climbed to 4,500 on a heading direct to Petaluma.
About eight minutes into the flight (from take off), suddenly but not sudden
like a switch or electrical problem I would guess power output dropped to
around 30% with a decent engine vibration. It felt and acted as if two
sparkplug wires were simultaneously pulled from the plugs. I noticed the EGT
that I had leaned to 1320 F was now down to below 1000 F. I checked the mags
and there was no difference in operation between both, 1 or 2. I pushed the
throttle in full with no effect. I switched tanks to the right wing with no
effect. I turned on the boost pump with no effect. I opened the power boost
which bypasses the air filter which resulted in a tiny bit of improvement
maybe getting me up to 35% power. I moved the mixture in full rich and it
smoothed out considerably and I would guess power output moved up to say 45%.
My speed leveled off at around 124 knots from the 151 it was just prior. I
was able to hold 4,500 feet. EGT rose only about to 1100 F. I contacted
NorCal and advised them of my situation. They were great and offered vectors
to Modesto that I was right on top of according to the GPS. I advised them
that it was totally socked in fog that spread about 20 miles in all
directions. After about 30 seconds later I slowly moved the mixture out and
it began running extremely rough with no increase in EGT so I pushed it back
in scared that I would make it worse and loose my 45% power output. NorCal
asked what I wanted to do and as airspeed was being held, it was producing
about 45% power and I was holding altitude I told them I would forge ahead. I
felt this was less risky than dropping through the fog layer into 3 miles of
mist visibility with engine problems trying to find a runway with towers up
around 500 feet in the area. Time is strange and I'm guessing after about 5
minutes of this, I felt the plane pull forward like there was a significant
power increase and I noticed the EGT move up to 1190 F. I leaned it a little
and EGT went up to 1200 F max and quickly began to run rough so I slid it back
in to full rich. After about 3 more minutes I would say the power seemed to
up around 60% and I tried leaning again. This time it leaned like always
rising to a maximum of about 1410 F before running rough so I backed off to
run around 1310 F - normal operation. The whole event from the start of
something being wrong to full power being restored was I would say around 10
minutes. Maybe a little less. From that point on it was like nothing was
wrong. Remaining 30 minutes of the flight went without a hitch and the engine
responded just like it should and always has. I can only think that there was
some water stuck somewhere in the wing behind one of the baffles and the
engine just had to work through it. I can't think of anything else that could
have caused this behavior.

The following day I went out and did 10 sumps of each tank and the center
sump. None showed any sign of water. If it was a bad mag I would have
expected to loose the engine completely when I went to mag 1 or mag 2 but I
didn't. Only thing I can come up with is either water in the tank or an
obstruction in the line.

The engine problem would not have been so critical if I wasn't VFR on top.
Without an engine and VFR on top you have very little options.

Thanks for your opinions and advise. Please post any responses here.

Mooney M20F

PS. Other info is that fuel consumption was normal and I'm at just now at 6
qts of oil remaining after 16.5 hours since last oil change. So that works
out to be about 11.8 hours per qt. No other indications of problems. Except
of course the oil and gas dripping out of the plenum drain after stopping the
engine. Engine has about 80 hours since LMOH and has worked perfectly.