New Holland t5000 idle issue

[futbalistos123]

Hello, I bumped into a problem lately. I started having issue with t5050. My problem is that when I increase rpm and let of the gas immediately, idle rpm fluctuate for about 3 seconds and then settle or when there is a change in load. When I slowly decrease rpm, it doesn't do that. Could it be a problem with injection pump, overflow valve or an air leak at return line? Or maybe is it just some junk stuck in the fuel lines or at the bottom of the tank? I changed the fuel filter, but with no improvement, although I think it improved performance. Please, do you have any idea what it could be? Maybe fuel pump needs adjusting?? Thanks for reply