accelerater pedal stiff

Hi can any one help.
I have just purchased a 2003 ulysse 20jtd which has a problem with the accelerater pedal not operating corectly.
The pedal is very stiff to operate as if the air system is faulty i.e one of the valves not opening or closing .The car will accelerate o.k but i have to use excessive force to push it down.
If on the motorway and using the cruise control to step up the speed it all works o.k.

I hope someone can shed some light on this thanks for looking.

Hi,
Same problem as that, I oiled cable etc but still stiff, any ideas welcome,
T