97 Jaguar XJR 4 LItre Engine Misfire.

Hi, I have a 1997 4litre XJR/X300 that has covered 110,000 miles. It has straight 6 engine and is supercharged. Up until recently it has been fine, then whilst driving along it suddenly developed a misfire.
The engine management light doesn't come on, however the car will not drive without missing and trying to cut out as soon as you accelerate.
When in neutral revving the engine, the revs will increase under light pressure but under heavy pressure the engine management light comes on and the car begins to stall. If you rev it slowly above 4000 revs it will be fine under load. We have checked plugs and terminals, along with hoses etc but this has not cured the problem. We have alkso tested a dtfferent air flow meter.