T-reg Vectra engine hesitating - advice required ?

Hi,
I have an issue with my T reg vauxhall vectra estate (1.8l) It has done around 100K miles and was serviced last year by Vauxhall.
The problem started off as intermittent and has grown over around 10 weeks. When I try to accelerate at a reasonable rate or change gear up at revs less than 4k, the engine starts hesitating / holding back and the car shakes.
I have changed the plugs, air and fuel filters. The ignition is on a low tension system so I can't change HT leads or distributor cap. The AA have looked at it to no avail.
The issue can just about be avoided by making sure the revs are over 4k when changing gear and keeping them high when releasing the clutch into the new gear, and by accelerating at the lowest possible rate.
When in neutral, the engine revs up with no problems at all, the issue only presents it self when travelling.
The engine computer has not logged any issues; the AA do not have this on the common fault list for the car.

Any advice anyone can give as to what the potential causes could be would be greatly appreciated. I am capable of doing most repairs/ checking that do not involve specialist tools but am currently unemployed due to the recession so am not able to put the car in a garage for them to locate the fault.

Many thanks in advance

Ash