Civic ignition timing is backwards

This is a 1995 Civic VX, 1.5 liter vtec-E, 5-speed manual, 174,000 miles. Long, long story coming up, so please bear with me.

For over a year I had a worsening steady speed miss after warmup (ran fine in open loop). It always smoothed out when accelerating, so I suspected the oxygen sensor (it was still the original) was sluggish, and running too lean when the engine was in lean-burn.

Before I got around to changing the O2S, the coil and igniter failed. Had it towed to a shop and they changed coil and igniter. From that day forward the car also ran poorly under acceleleration when warm.

Replaced 02 sensor, cap, rotor, plugs. It ran much better, but still had a slight stumble on acceleration. This stumble gradually worsened over about 6 weeks.

Now it runs poorly under load, with a very noticable miss and slugghishness on acceleration and hill climbing. If I jab the throttle at idle, the engine coughs; I can hear the miss in the exhaust.

All this happens AFTER warmup. Again, it seems to run fine when in open loop.

The CEL is NOT on, and NO trouble codes have been set except from my experiments with disconnecting sensors.

Plugs were black with carbon after only 2000 miles. Replaced with fresh dealer plugs of correct type.

There's a bit of carbon on the base of the oxygen sensor. (Hope it's not fouled - $$$)

Here's where it gets weird.

I got out the timing light and found that the ignition timing is being RETARDED about 10-15 degrees when I blip the throttle. I've never seen another engine that had its timing retarded when the throttle was opened. I can't find another VX to compare, but a friend's '93 Civic DX advances quite a bit with the same conditions.

After this initial retard when throttle is snapped open, timing quickly falls back to the base value (16 deg BTC), and advances only slightly with slowly increasing RPM.

I suspect the "backwards" and sluggish timing is what's causing the poor driveability, but I can't figure out why the timing is acting weird.

Here's what I've tried so far -

1. Replaced MAP sensor w/ aftermarket part. No change.

2. Checked coolant temp sensor by Haynes manual method - tested OK.

3. Checked the CKP, CYP and TDC sensors in the distributor, also Haynes manual method (resistance) - tested OK.

4. Checked TPS with analog ohmmeter - resistance varied smoothly, with no abrupt changes over the range (I think max was ~5000 ohms).

5. Tried another (used) ECM - no change.

6. Did the vacuum test for a plugged cat - passed.

7. Tried disconnecting / plugging vacuum from EGR valve - no change.

8. Tested compression - 210/210/210/220.

I can't think of anything else that would cause such strange ignition timing behavior.

Can you help?

Thanks!