Car shakes and rumbles, dodgy 4th gear

[zaz96]

I've got a VT S1 Automatic v6.

The car rumbles a lot which seems to be coming from the gearbox, the car plays up in 4th. It will click into 4th gear but only if I keep the revs at 2000rpm, if I go any higher, it will go back into 3rd to about 3500-4000rpm.

What's wrong?

[Tree cutter]

You could try checking the fault codes to see if there is an electrical fault but it sounds a bit like a job for a transmission specialist.