Mine is not running on 3 cyclinders, but once i had that problem. I was going an uphill and I inserted the 3rd gear and the car strated knocking terribly. I thought that I inserted 5th by mistake but the problem was that it was running on 3 cyclinders. I went to my mechanic and he removed me one of the spark lead's suppressor. and it worked fine. everything else, fuel filter etc should be ok because I just serviced it about 2months ago, complete service, oil, oil filter, fuel filter, timing belt ...
I think I have to try changing the plugs first and see if it gets any better.
The problem really is in between 1000 and 2000 rpm I think, somewhere in between those revs, but it does not become so rough as it was when it was running on 3 cyclinders, but not as smooth as usual. I thought it needed a clean!
Sorry for my $hitty English, but I am not from England
