adding a return spring on the pedal will cure it, the TB return spring seems to loose its tension for the last fraction with age, so we just add another light spring to the pedal as that seems to cure it more than anything else.
To check, just get the car into its high idle situation then put your finger behind the throttle pedal, you should only have to very lightly 'lift' on the pedal to bring the idle down. If it does then you can add the spring no worries.
If your wondering what type of spring to use, look at the clutch mechanism ratchet spring.