RPM SURGE - 90 Eclipse 2.0 Non-Turbo

Installed low-mileage engine from engine importer. (Approx. 35-55K miles) Swapped original intake w/all related attachments/sensors etc, exhaust manifold, and remaining original sensors from previous engine. Starts and runs fine(1800-initial, drops to 1500rpm within 3 minutes) until closed-loop operation at which time engine begins to surge between 1200-1400 at approx 8 min from start-up. At approx 15 minutes (normal op temp) the engine will go into a die/recover cycle, also with a 200rpm bandwidth (<1 sec intervals). When the engine dies and recovers, I can feel it torque then relax (a fairly good work-out for the mounts). Bring the throttle just off idle and it returns to a nice rolling 100rpm surge @ 4 second intervals. Above 2000=50rpm surge w/same 4 sec frequency. Turn the A/C on at idle and idle drops to 900 w/50rpm surge @ 4 sec interval (barely noticeable). In drive: idles @ 800 w/30rpm variation (not cyclical). If I just turn on all electrical accessories; Headlights, Rear defroster, blower fan etc, and with the cooling fan on, it will idle steady. As I add the loads one by one, the cycle time to 1400rpm lengthens until sufficient load has been achieved to maintain steady idle. I watched the timing marks during this whole process. They toggle between 8 degrees and about 20 (switching like a transistor).

I installed a new timing belt, checked and rechecked the marks three times after two complete cycle rotations.

The cam sensor can be installed in two positions; Correct & 180 off. I was reasonably careful to watch for correct positioning. I'm not sure if it would even run if it were 180 off. (I turned the CMP sensor 180 and it runs exactly the same)

I checked & adjusted the timing according to specs, but I couldn't get any power out of it under load until rpm gets above 2000.

Throttle snap from idle produces slow response and an occassional backfire.

The O2 sensor may be a little slow (switches from 200mv to 800 in a second or so), but it doesn't seem like it could cause this symptom.

I tested some of the other sensors, but couldn't find anything out of specs.

Symptom Summary:

Idle @ normal op temp:

No loads = Cyclical 200rpm surge
(die/recover; like fuel shut off)
@ <1 second intervals.


w/12v loads = Die/Recover
200rpm cycle w/increasing recovery
time to 1400rpm until most or all
loads on which produce smoothe
idle.

w/ only A/C on = 900rpm w/50rpm
rolling surge @ 4 sec
intervals

Drive = 800rpm w/30rpm non-
cyclical variation

OFF IDLE in Neutral :

1500rpm : rolling 100rpm
surge @ 4 sec

>2000rpm : rolling 50 rpm
surge @ 4 sec

Have you ran across this type of problem and if so, could you suggest anything I might try to locate the cause?