[16v] DPR/mixture/timing questions

[doubt at inwave.com (Nate Mellom)]

I've been trying to solve my low rpm bog/hesitation that has been 
plaguing my '88 since I got it.  It'll pull and rev okay when cold, but 
as soon as it warms up it has no power at low revs, and if I try and 
accelerate it bogs down so much it almost dies.  I have to repeatedly 
bounce the throttle for the revs to get up so it has enough power to move.

Some background:
The PO was never able to get it running right, and has a bunch of 
receipts for work done, including a new distributor.  He said the timing 
was spot on. I've checked all the common culprits, vacuum leaks, 
grounds, etc.  I installed a new O2 sensor, the old one was a BITCH to 
get out, obviously it hadn't been replaced in a long, long time.  No 
difference, so I made up a DPR test rig, and found (once I got 
everything set up correctly) that the mixture was turned to full rich. 
I would assume the shop that did the work forgot about the O2 sensor, 
couldn't get it to run right, and upped the mix to make it get home. 
Anywho, I was bouncing off 5mA at idle, and all of a sudden I lost the 
signal.  The engine was fully warmed up, and I had been working on it 
for probably 30 minutes.  If I revved the engine I would get a milliamp 
spike, but it would always settle to 0 at idle.  I have a replacement 
DPR I'm going to throw in, but what would cause a sudden loss of signal 
from the DPR?

I'm also going to assume that since the mixture was so far off, and that 
the O2 sensor was bad, that when they replaced the distributor the 
timing was set well enough to make it run, poorly, but run.  I don't 
have an adjustable advance timing light, but a friend does, but is it 
necessary to get timing set on a 16v?

I'm going to install the new DPR and check the mixture again, I've had a 
sneaking suspicion that it's been bad or failing since I got the car.

Anything else I may be missing?

Nate