A slight vibration through the gear lever has developed over time into a very lumpy running engine.
The engine starts with no problem at all, from cold or hot. At idle the car feels like it is running on 3 cylinders, if the accelerator is depressed the engine picks up and runs more smoothly, however the 3 cylinder feeling returns during constant throttle. The situation remains when the engine is hot.
Whilst I do use the car for short trips occasionally, usage is predominantly longer runs, motorway / dual carriageway
I have ordered a diagnostic cable, which will hopefully narrow the search, In the meantime any suggestions as to likely causes would be much appreciated.