Revving in idle and losing power when driving

Hey, bought this car on Thursday, driven it since (including this morning) with no problem. Got in it this afternoon and when I started it, I noticed that it was revving itself, between about 500 and 1500... Thought this was odd but, decided to head off... Within seconds the car lost power, foot to the floor and nothing! Tried dropping a gear and it jerked forward then did the same again. Turned it and drove back to the house, parked outside and left it running and it was still revving... Switched it off and contacted my cousin, her husband fixes my cars. While he was on the phone I started the car and it was idling normally! Didn't attempt to drive it though, waiting for him to come look at it, any ideas? Have searched and seen lots of similar problems with lots of different answers!!