Drove my van to work today for the first time in a few weeks (been
enjoying my TDI passat) and noticed a flat spot in the power curve at
about 4000rpm.  I think I might have noticed this in the past, but can't
quite remember and certainly didn't remember it being this flat.  As I
floor it and let it rev. out power climbs nicely through the 3000's but
then sort of hits a wall at 4000.  It's really quite noticeable, almost
as if there is some extra load suddenly being pulled and the motor can
no longer gain revs. as easily.  There is no missing or stumbling.  I
imagine this is probably some sort of retarding of ignition timing
effect, but is it normal?  The last time I set the ignition timing
(probably a year or two ago) I just did it by feel and, as I recall,
advanced as much as possible until onset of pre-ignition and then backed
off just enough to stop the knock at full throttle.  Could this method
cause such a flat spot to occur?

Thanks,
Mark