Lack of power/unpredictable power

[photo@intrex.net (Bill Collins)]

I'm having an interesting problem with my '90 Cabriolet (8v 2H engine with Digifant).  Here's some background:

Recently I changed the timing belt and valve stem seals.  After this the car behaved strangely.  Sometimes it would run fine, other times it would accelerate normally up to about 4500 RPM, hit a "flat spot" until about 5500, then rev happily to redline.  At other times, the acceleration would be weak and uneven (almost like I was keeping time to the radio with my right foot)  On a few occasions, I could hardly keep the car moving. Sometimes it seems to accelerate a bit just as I let off the gas.

Here's what I have done to try to fix it:
- Rechecked ignition and cam timing
- Replaced the air intake boot (to fix a significant hole)
- Looked for other vacuum leaks
- Checked all the ground wires and vacuum lines that were disturbed in doing the work on the car

BTW, its quite a bit better after fixing the hole in the intake.

My instincts tell me that its a fuel mixture problem of some sort. (possibly a lean mixture causing pinging which causes the digifant to retard the timing, although I don't hear any pinging)

Here's what I plan to do this weekend:

- Look for vacuum leaks again
- Disconnect the O2 sensor
- If it still runs bad with the O2 sensor disconnected, use the O2 sensor and voltmeter to monitor the mixture while driving.
- Compression test

Any other ideas?

Bill