After a while the car starts to stop revving and loose power? on 2000 Volvo S80

It's a 2000 Volvo s80 and when first started runs smooth but won't rev up at all after running for a bit it gets a rough lower idle but will rev up to 1500-2000rpm's and occasionally it will start and run alright for a little while, what's causing this?

Asked by for the 2000 Volvo S80
scan codes and post if any
2 more answers
scan codes and post if any
Check your fuel filter and fuel pump.