A few weeks ago I got stuck in some road works and the speed limit was 40, to I was crawling along at 40 behind this car and I noticed my Focus was juddering a little, almost like I was putting my foot on the accelerator and lifting it off in a split second, I was in 4th gear at the time and doing about 2,000 revs.
So I put it into third to see if it would still do it, and it did, all the time pretty much between 40 and 45 it was juddering.
After about 10 miles or so the engine warning light came on, then a couple of miles later the roadworks ended and I was able to speed up and it was fine again. I thought at the time it was just maybe some dodgy fuel going through or something.
Tonight I was coming back home and got stuck behind a lorry which was doing about 40-45, and again I noticed it.
I've tried to replicate it at other speeds and in other gears (I've cruised along in 5th at about 2,000 revs no problem, same with 6th), but it's always when I am cruising along between about 40 and 45 in 4th, then if I change to 3rd it still does it.
Any ideas what on earth might be causing this? Many thanks in advance.