Annoying miss. 1990 RR - 3.9litre

I have this intermittent engine miss. May happen once a day over a total of
80 kilometers of travelling. Feels like a 80% loss of power. Engine will
always keep running and not stall. If I stop the vehicle, the engine idles
nicely and I can rev freely up to [say] 5000rpms. If I drive away then the
bad missing will still be there. Keep driving and it appears to go away
within a few minutes of starting. Mechanic cannot find the cause; have
swapped out the Hot Wire Air Flow Sensor, Computer, Throttle Position
Sensor, Water Temp and Fuel Temperature sensors have been checked, Spark
Plugs and leads are new, new Petrol Filter and Air Filter.

Also at times there can be a very slight miss around 2000rpms in top gear on
a level road, it to can disappear within 30 seconds of starting.

Your comments please?