156 2002, 2.0 JTS petrol. I (the car)had a splutter/cough a year or so ago
and it eventually went away on its own. Its come back now and I would like
to at least figure out what is causing it, if not fix it. The problem:

Its independent of gear (done it in 3rd, 4th 5th). I don't tend to rev over
about 4000 most of the time as its a nice torquey engine. Problem occurs
under load, usually at around 2000 revs. It is repeatable - the 270 degree
slip road onto the M8 at the top of Livingston is about a 50mph corner, 5th
gear. As I come out of it I speed up to er... 70 (ho humm!) and it will
splutter most times there. It also does it in 3rd coming out of the silly
little roundabouts they stick along in-town dual carriageway 40mph
stretches. Its a cough/splutter, lack of power, followed by 'beep beep'
motor control failure - go to dealer. As I feel the splutter starting, if I
back off on the accellerator and just speed up more gradually all is well.
If I drop a cog and increase the revs, all is fine.

I have ruled out dirty fuel as its been through several fill ups.
I have rules out engine temperature as it doesn't make any difference.
I have ruled out spark plugs and electrics because the revs are low, so the
demand is not so much - wouldn't this cause problems at higher revs when
there are more sparks per minute?
I have rules out air flow for the same reason - the engine isn't getting
through as much air as when the revs are higher.

This (in my mind) leaves the knocc sensor, or something else.

Any ideas / suggestions?

GT