The timing belt - sort of. Loss of power in 2 steps last week turned out to be due to the t-belt slipping about 4 teeth. Original posts describing the problem are referenced below.

So on a well-maintained engine with a regularly re-tensioned timing belt, why would the belt jump a few teeth? The shop thinks it's the aluminum IPD adjustable cam gear - over the phone, the tech said its teeth are "incredibly worn" and they're throwing a used steel gear on in its place. Until I see the old gear, I'll reserve judgement. The belt was close to being replaced (it's at 50K right now), maybe the belt wore prematurely because of the AZ desert heat, maybe the high revs which the VX cam beckon me to enjoy helped as well.

So once I retrieve the car & cam gear, there will be some measurements made, and possibly a heart-to-heart discussion with IPD. More than likely I'll be looking to modify a steel cam gear to throw in a 5 degree advance
which I dialed in via the adjustable gear.

http://www.brickboard.com/RWD/index.htm?id=698376

http://www.brickboard.com/RWD/index.htm?id=698493