'87 5KCDTQ - stumbling under throttle

[George Harris]

The patient: '87 5000CDTQ with 343,000km
The symptoms:
A few weeks ago the car started having problems idling when cold. No problem idling when hot.
Then 2 weeks ago it started running rough at higher speeds. Mostly when accelerating, it appears to lose power at higher rpms, say about 4,000 in first, down to 3200 in 4th. It appears to cut out at about 11psi of boost. When cruising it seems to be very smooth, except it will seem to stumble. 

What I have tried so far:
- Checked the plugs - they are all consistently white /grey, new last year.
- Wires, cap and rotor all new last year, and no visible cracks.
- Changed the fuel filter.
- Removed and washed the Michelin man and checked for cracks. - None.
- Removed and cleaned the ISV. Plugged it in and turned on the ignition and it vibrated.

Car does run better after all that but not quite right yet. Any ideas?

Thanks
George