fiat punto gt2 inlet hose and car running sick?

hi there guys my mate has just bought a punto gt2 which is running rough,when it gets to 3-4 thousand revs it seems to splutter and wont go over 4000 rpm when driven however when car is stationary it will rev up to limiter!
the car has a front mounted intercooler,ind kit and a decat
its has had a new air flow meter,plugs,leads,coil pack,temp sensor,new fuel filter etc to try to solve problem but still the same.
yesterday i was under the bonnet and saw a hose (located on gearbox side of inlet manifold) approx 48mm dia not conected to inlet,the hose its self has been capped off further down the hose.anybody got any ideas what this hose is and where it goes to further down the engine,
cheers dave

Follow that pipe to the other end and tell us what you find.

i followed hose from where it was supposed to be conected to inlet manifold down to the other end of the hose and it was capped off.the car has a few hoses that have been blocked (bodged)off by previous owners in past
all my mate wants to do is getting all bits back to std and get it running as it should so finding where this hose goes would be a start

Sounds like you need a manual for the car.