Accelerated off an island onto a dual carriageway this morning and once in 5th gear it wouldn't go any faster than 55mph. I floored it to see if it would change down a gear but it wouldn't. So I manually knocked it down to 4th which would lower the revs but would not let me go any faster. I pulled over and turned it off for 5 mins. Started it up and then it was fine all the way to work which was about 15 miles away. It let me drive upto any speed no probs. There is no turbo whistle, no smoke, no dash lights/messages etc. Any ideas what this could be? Just a one-off glitch, or signs of something bad coming my way Any advice would be great thanks.