1999 Ford KA hig revs when stopped and power loss and kangarooing

Hi, please help!! My car has been revving really highly when I slow down (from any speed) and when I'm stopped. Eg. at traffic lights the revs will stay high even if the car has been taken out of gear and foot is not on the accelerator. It has also started losing power when I'm driving, it'll just randomly drop power with a jerk and slow down, then it'll suddenly come back again. There's no pattern with changing gears or anything. It's also struggling to get up speed, the last time I drove it I couldn't get to 60mph. I've not driven it since as it feels as though it could die at any point and therefore not safe! There's a new air filter in there, all liquids are topped up, had spark plugs and leads winter last year. Have been advised by a friend it could be the air mass filter???? Any thoughts would really be appreciated! Thank you