HELP HELP HELP

Vectra exclusive 1.8 55 plate, has a low rev judder/misfire.
its had the following changed in a bid to stop it.

New Plugs,
New filters,
stripped and cleaned throttle body.

the judder happens around 1500rev, at low speed. higher p the revs, it never misses a beat.
nothing in the engine management etc, so all sensors seem ok ,

putting it bluntly, im stumped as to what to do next.