I have a 2006 warrior Auto that I have been having the same problem with for the last 6 months. Starts fine but runs lumpy. Revs are slow to rise until reach over 2500 then seems to clear and be ok. However, if you put it in drive the revs won't rise at all.

I have had 2 suction control valves and MAF sensor replaced which seem to clear it for a while then the problem comes back. There are no fault codes stored.

Where do I go next?

early autos have been known to have problems with the loom for the auto box rubbing on the chassis
have a check for any broken/rubbed wiring where it's attached along the cross member etc