Mondeo 1800 1993 - lack of power and lots of judder?

I have a 1993 Mondeo which is having a few problems. It starts up fine but after a few minutes on the road it gets a bad judder (the old kangaroo juice syndrome) - this only last about 5 - 10 mins then its fine. The other things is that after this has stopped i have very reduced power - I feel like pushing my feet through the floor to run to get it to go faster. lolll A lil like the flintstones. Once it goes its fine but it takes soooo long to get there im sure this isn't right. I have changed the plugs, air filter, oil and replaced the front section of the exhaust (cat had gone). Any suggestions??????