Originally Posted by buchnerj

So your saying that the timing belt skipped a tooth? Are you positive there was no valve damage? It seems odd that the compression is low in one cylinder and also that you found the timing off by a whole tooth and nothing happened. You would be lucky. If you already have things apart, I would pull the head and inspect the cylinder with the low pressure. You seem to have run down all the usual suspects, so it is time to dig in deep and tear her apart.

The usual culprits would be:
1) boost leak (turbo issue)
2) fuel (fuel itself, too) and/or boost cut
3) bad fuel pump or relay
4) bad injector or coil pack (spark plug included)

Things you have to now rule out:
1) Valve damage (springs included).
2) Piston/ring condition

Those are all I can think of off the top of my head. Your best bet is to pull it apart and see what is going on in that cylinder with the low compression. It is a tedious task, but might be necessary in your case.

EDIT: Also try the oil test. See if putting a little motor oil into the cylinder with low compression helps raise it up to the level of the other three. If this is the case you can rule out valve damage and narrow it down to the rings/pistons. If not, than you can be pretty certain that something happened to the head.

Hope this helps, and post up what you find!