86 16V Scirocco - Off / on rough idle, stubles on low rpm - runs fine higher rpm, looks rich at idle?

[Road America]

120 mile road trip yesterday, in the last 30 or so miles, I came to a corner, while acceleration out from about 2,500 it felt really down on power then all of a sudden as engine speed increased it came back.

I slowed down and tested this over and over, very consistent. At lower RPM's it stumbles and is down on power (both under a load or revving the engine in neutral).
Idle goes from perfect to rough, no real pattern or consistency.

Idle speed is fine.

Back home in the garage while it was idling, I poked around the engine for any obvious changes like vacuum lines or lose connections. I noticed black spots on the concrete behind the tailpipe, running rich (no CAT...)

I replaced the fuel pressure regulator in spring since it was leaking, it has been running perfect all summer. This came on all of a sudden.

Not even sure where to start, very much a CIS noob.

Thanks

[ziddey]

see sticky for guides on checking dpr current and associated tuning.