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Ok folks, got a real head scratcher here concerning idle and ignition. My son's 89 MJ now has a 99 XJ driveline and axles. The only thing that is 89 is the sheet metal, so we are basically looking at a OBDII system. The problem is the engine idle becomes erratic at random times - there is no rhyme or reason to when it happens: at cold start, after running for a couple of minutes, after running for 1/2 an hour, after 5 times around the block - you get the idea. It happens when both at idle or in gear...driving or not. The engine always starts immediately upon turning the key, and when it runs correctly it is dead nuts on 700 RPM and smooth as silk. When it starts the crap, it hesitates several times, then the MIL lights with the two codes, and the idle varies between 350 and 1000 RPM, jumping up and down irregularly. The codes are P0340 - no cam sensor signal and P1391 - erratic cam sensor signal. We have exhaustively worked this problem (for about 4 weeks), and it is truly spoiling his conversion experience. What compounds the issue is the engine ran without this problem in the donor before being rebuilt, so whatever it is, we did it. So far we have: Checked all sensors. Replaced the cam sensor x 2, with NAPA (Echlin). Replaced the crank sensor x 2, with NAPA (Wells). Completely removed the front of the engine and confirmed the timing chain is correct and sprocket marks are in time. Replaced the spark plugs x 2 with champion plugs gapped at .035. Replaced the rotor and cap (Mopar). Replaced the plug wires (Mopar). Replaced the Distributor (NAPA). Removed he valve cover and watched while turning harmonic balancer by hand to ensure the valves are in time with TDC (as piston # 1 is coming up and the rotor is approaching 5:00 O'clock the intake valve is closing; then intake valve closes and as rotor passes 5:00 O'clock the exhaust valve begins to open). Replaced both O2 sensors. Replaced the IAC. Replaced the TPS. Ohmed out the wires from the sensors to the PCM. Pulled the wiring harness and removed all the loom, checked the wires for melted or torn insulation. Did find two of the cam wires had damage, soldered and shrink - tubed the repairs - re-ohmed wires. Replaced PCM with rebuilt one (but removed and sent rebuilt one back when it did not solve problem). Checked the running voltages at both the cam and the CPS - dead nuts 5V. "Cleared" the PCM capacitors by touching the (disconnected) battery cables together for 30 seconds. Used smoke machine to check for vacuum leaks and found none. Hooked up oscilloscope to check square wave pattern for faults on both the cam and the crank sensors. When checked, they are perfectly in sync while the engine is running fine but when the engine starts it's crap they go out of sync. (for those who wonder, the in-sync distributor is 2 degrees out at idle...when the behavior I describe starts the distributor goes instantly to 30 degrees out). I have read that the distributor hold-down fork should not be modified in any way as the ignition timing is controlled by the PCM, but when taken to a local garage (that I trust), the owner states he has heard that modifying the fork (to be able to turn the distributor) has resolved issues such as this....but before I do this I'd like to get input from you guys about the problem. Any and all ideas are welcome....because we are out of options. Thanks