Check your induction hoses, the ones that run from your airbox to the carb. One may have a split in it somewhere, this increases the airflow into the engine which causes the ECU to add more fuel to the excessive air, hence your revs rise.
Its a common problem with Fuel Injected cars when the hoses start to age.
Uneven idle speeds can be also caused by a faulty MAF sensor, again the is is connected on the induction before your carb, it will have a wiring plug going to it, remove it and clean it with brake cleaner but be carefull as they tend to be fragile.
The return spring for the throttle may be stuck but i doubt it with the explanation you gave.
Zippy