vauxhall astra 2004 1.6i won't rev above 3500 when warm, NO engine management light o

Hello,

I have 2004 1.6 petrol vauxhall astra, for the last couple of weeks I've had this same problem. When I drive the car in the morning it's drives and revs with no problems. Once the engine is warm after 10 minutes or so the car won't rev above 3500rmp and is very sluggish. There is no engine management light on so I can't see it being a sensor. Can anyone help??? Ben