97 2.0 GLS OMEGA STALLING?

My omega has just started to run irratically when the reves reach about 3000 rpm it idles fine but when truning the car etc the revs drop and the car stalls. There is a light on the dashboard which looks like a gearbox (but cant find anything, anywhere which explains what the warning lights are for). The rocker cover gasket was replaced about 1000 miles ago and the leads were replaced at the same time. Can anybody please help me.

Thanks

Chris