I finally managed to get a scan tool on my 99 454 mag mpi....A quick run-down of what it has been doing all summer long.....Runs great most of the time, but other times, it will just fall off RPM's, (engine won't die). Then will struggle to get over 2000 RPM's. If I let it sit for a few mins, it will run fine...Things done so far...All new sensors, fuel pump, regulator, I've went thru the pick-up tube & vent, & a complete tune-up....Now the results.
Nothing definete, but one thing that was odd, the timing was all over the place. It would go from -5 to +18, it does this constantly, & regardless of the RPM's. I checked for a loose timing chain, by watching the rotor button while barring the engine over, that seems fine. I'm leaning towards the ignition module (distributer mounted)....Does this seem to be a good direction, or...once again, am I way off base???

What scan tool are you using?

I can't tell you more than this, the logo on the top of the tool was "Quicksilver". I had a mechanic run the scan, & he seems to think that the module is a good direction to go. He ran a pretty detailed scan & found nothing out of the norm, except the timing being all over the place.
I know from my personal experience with street rods, even a radical cam'd engine...the timing will smooth out when the RPM's go up (that is watching a timing light). So, I did find it quite odd, that the timing would never smooth out.

I would agree with the module being the next place to check.

Thanks for the input, I'll be ordering one today.

It is normal for the timing to be erratic at idle, it should smooth out as the RPMs increase. Spark control module could be bad or the ground is faulty.

if that has a knock sensor the computor will put the timiing all over the place if the sensor is getting hit just a thought good luck

I did remove all the grounds & cleaned them real good. All were tight & I didn't find any corrision, but however, when I removed the module from the distributer, I noticed a white residue all over the mounting plate, & even under the module. I was able to wipe it off, just not sure if it was some sort of dielectric grease or corrision preventive. I do know that I should put the grease under the module, but should I also cover the entire mounting plate?
As for the knock sensor....It does have one, but the scan tool showed that there was no "signal" that was indicating a knock. So I would assume that the sensor is working correctly?

The ecm is moving timing all over the place to keep the motor's rpm constant. Since it is quicker to change the timming than to have the IAC change.

to check the timing at idle you need to put the motor into timing mode, rinda has the tool that will do this, about 40 dollars.