For the last couple of months, I have been having performance problems with my car.

Facts:

- Just turned 90000 miles.
- Timing belt and related done at 75000 last year.
- Spark plugs just replaced (NGK BKR7E-IX) which seems to have corrected some annoying roughness I had been experiencing, and literally added 2 MPG to the average.
- Fuel filter replaced at 80000 and then at 87000.
- Car has Eurocharged tune as of end of last year. Problem affects both tuned and non-tuned (stock program) operation.
- Car has aftermarket intercoolers because factory Volkswagen units leaked.
- No relevant codes on VCDS.
- No OBD2 codes at all.
- MAFs and TB perfectly clean.
- Factory SE exhaust.
- Air filters replaced 5000 miles ago.

Symptoms:

- Steadily slowed down over the last 3 months.
- Runs great cold. Start it, drive moderately for a few minutes until oil temp is showing something, and it's fast as normal. Let it heat up, it slows way down.
- Problem is intermittent but increasing. So far it has avoided my data logging efforts.
- I suspect a problem on the left turbo due to fairly significant (4 ounces in 15000 miles) oil in the intercooler while the right side has a few drops or a film.
- Strange power on/off thing going on at highway speed. Take an onramp at full throttle from a stop, boost comes in strangely abruptly and in "stages". If you stay on it, it falls on its face around 110 and feels as if the low-fuel "limp" mode has activated. Stay into it for a couple of more seconds and sometimes power comes right back to what it was before.
- Turbos sound "sick", whistle-ish and strangely noisy, rather than smooth and "melodious" like they used to.
- Almost feels like when my friend's Subaru was hitting overboost and it would cut his boost level way down for a few seconds and then give power back until it hit overboost again. Problem is that my slowdowns aren't preceded by increased power.


I have tried logging everything like MAP requested vs actual and a few other things. So far, the problem has not happened with the computer connected.

Any ideas where to start testing next?