I have a 92 200TE wagon with 100k on the clock. It is giving an intermittent hesitation problem when accelerating. Sometimes the car responds immediatley, sometimes it takes a few seconds. This problem is evident both when accelerating from a standing start or when the car is rolling at speed. Car also starts off with revs at about 850RPM but soon drop after about 5 mins to 500RPM and is quite rough at idle.

Work Done.
Auto gearbox checked and does not appear to be at fault
Carburettor & fuel injection system checked - no faults identified
Exhaust - Catalytic converter broken up inside. Car does not require a Cat so Cleaned out but - no affect.
70 degree Vacuum Value & Breather hose replaced - no affect.
Replaced new Bosch distributor cap and rotor - no affect

The car is the old 2.0L engine and has no air flow meter.

I have exhausted all the specialist mechanics. Please let me know if you have any ideas at all that may be causing this.