It's been a while but still have not been able to resolve the ongoing idle stumble most noticeable when in gear and standing at a stop, it is an irregular blip that happens regularly, like skipping a heartbeat. Below is a list of things that were done and as such are being "temporarily" ruled out-of-scope as sources for the stumble, although open to ideas and suggestions.
Things done before I purchased the car:
New EA, (I ran diagnostics and opened it up and it checks out good)
New upper main wiring harness
Newish fuel pressure regulator (I tested for holding vac and no fuel in line)
Things I have done within the past 8 months:
New lower wiring harness and Bat cable
New ECU
New Spark plugs, gapped (Bosch copper, the correct ones)
New spark plug wiring and stubs
New coils (all 3)
Have run a few bottles of Lubro Molly valve and fuel injector cleaner
Cleaned out the EGR lines (were not very dirty or clogged)
Cleaned out and restored purge valve (MOT) yesterday, holds closed now was frozen open before, this helped a bit
Looked for other vac leaks and cant find any, my idle is about 500-550rpm so it does not seem abnormal. I have no CE light or codes on any of the three main DM leads.
Given that it has a regular pattern to the idle stumble I imagine the issue is electrical, possibly on the spark side but also on the fuel end like an injector being switched off momentarily.
Things that I have yet to do, but could possibly be the cause of the problem:
Fuel filter (will change soon)
O2 Sensors, have not replaced and not sure if original
Fuel Injectors are original and have not seen any signs of leakage, they are all tight and I'd be surprised if they have failed or are sticking and would have expected a more sustained running problem as a result rather than a skip.
Any suggestions?
TIA