putts

Hello everyone. I have a 2000 model fazer 600. It has started running on 2 sometimes 3 cyclinders,it has done 27k miles and up until recently ran perfect. I tried changing the plugs, it ran fine for 10 seconds then back to 2/3 cyclinders again. I also tried swapping the plug caps but it still ran on 2, with cylinders 2 and 4 not firing. Anyone any ideas on what to do next ? Thanks in advance Mark