w/engine off, have someone press the pedal while you confirm that the throttle plate opens completely. See if there is any binding while other pedals pressed/trans in~out of gear etc.
Disconnect the throttle cable and move the plate by hand full range...see if it sticks. Check to see if it moves laterally on the pivot bearings, etc. Disconnect the intake tube and check inside with a light for any signs of sticking/rubbing.
If the plate moves free and the pivot shaft bearings are snug, check the pedal/cable action while still disconnected for free movement, etc.
Try to get your hand on the throttle cable at the back of the engine where it runs along the firewall...is the rubber covering still intact? Is there any polishing of the paint, etc. on the firewall that may indicate wear/binding over the years?