Sticky throttle

[el roacho]

Sometimes when my engine is cold and I release the throttle and pop in the clutch to shift up a gear the revs keep going up sometimes to over 5000rpm so I have to do the hell quick gear changes so I don't damage my engine. This problem only occasionally happens but always when cold and it happens more often on wet days and the accelerator pedal actually feels like its not coming back as much as it should. I tried spraying a bit of oil in the throttle body but I can't get any oil onto the throttle cable as it's got a plastic tube around it. Any ideas on how to fix the problem?