It is Morning Sickness... Sticking exhaust valves from carbon deposits
and lead salts - a Lycoming special... A mag being off timing is not
going to drop the manifold pressure to 20 inches at part throttle, but
3 valves stuck open to the great outdoors will, in a heartbeat...
After you shut down and let it sit the heat build up in the heads
expands the valve bore and they finally start "poppeting" again...

Try putting AVBLEND in the oil and add MMO to the gas (4 ounces per ten
gallons) and take it out and fly it right away.. GIve it a good long
run at high power settings... WIth a bit of luck this will begin
dissolving the lead and carbon deposits in the valve guide and you are
home free... If it persists then you need to pull the valve cover(s),
alternately squirt CARB CLEANER and WD-40 (or Liquid Wrench and LPS #2)
etc., down the exhaust valve stem and immediately bounce the heck out
of the valve(s) with a wood block and a ball peen hammer.... Don't be
shy, make sure the piston is at the bottom of it's travel, and whack
the heck out of the valve, make it really dance ( I normally remove the
rocker arm to do this so I can put some muscle into it, but you can
leave the rocker arm on)... Chanting in tongues while you whack away
is highly recommended for the theraputic value...

denny