Ford Fiesta slows down and speeds up itself at 70mph - what's going on?

My Ford Fiesta diesel, which has done 80,000 miles, has stated acting odd. At around 70 mph I start gently rocking back and forth in the seat - it feels like the brakes are being applied gently. It's more obvious going uphill but this might just be because the loss of momentum is greater. Any idea what is the cause, and is it potentially dangerous?

Asked on 24 February 2014 by barnesboy

Answered by Honest John
Might be a driveshaft issue, best to get it checked by a local garage.
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