I would suspect a bent or sticking intake valve stem, which could be stiffer
when the engine is cold or been sitting for a while.

I think it is time to pull the head and do a valve inspection and grind and
reseat. At this time you should look for anything in the cylinder that could
be causing this. You could also see if the valve has been hitting the piston
top. I don't remember if these pistons have a indentation machined into the
top for valve clearance but if they do, it is possible that it is filled
with carbon deposit.

I'm looking at a MD2B piston here and it has no valve notches but does have
deep combustion chamber in (near) the middle of the piston tip. Could it be
possible that something has been sucked into the engine that is now riding
around in that chamber and is sometimes interfering with the valve head
clearance with the piston top.

This is a nice little engine and well worth the shop time to resolve this
problem.

--
My opinion and experience. FWIW

Steve
s/v Good Intentions