2000 Vauxhall Vectra engine power loss

vehicle struggles to start, must have accelerator fully depressed. Once started and running, suffers from power loss and if accelerator is pressed rapidly when driving, revs drop and engine begins to stall. but this doesn't occur when not in gear, it revs as normal. no warning lights on dash whatsoever, problem has occured in the last week. drives as normal other than this.