--Had what I suspect is the same prob with my new P-C compressor.
Turns out there's a nasty little "improvement" in the form of a
combination intake filter and check valve that screws into the cylinder
head; if it becomes clogged the force of the vacuum can suck parts of the
damned thing into the cylinder, causing scoring and all nature of things.
Somewhere in the manual, in a not terribly obvious place (it should be
inscribed in inch-high letters on the cover IMO!!) it says to replace it
every two months or so. They cost about $10 each, so add that to the
overall operating cost and suddenly the P-C ain't such a deal. I was
"lucky" enough to have mine fail while still under warranty, but I'll
NEVER buy Porter Cable again..

--
"Steamboat Ed" Haas : Yet another genius
Hacking the Trailing Edge! : without a job...
http://www.nmpproducts.com/intro.htm
---Decks a-wash in a sea of words---