Maybe dirt in the tank? Might explain why sometimes it's fine and sometimes not. Could get swirled around by bad roads/energetic driving.
Not sure what you could do about it, just thought I'd bounce an idea so people could discard it, or agree and suggest a solution.
think bazza might be right, ive got the same car and had the exactly the same problem a while back it only seemed to do it on the motorways on long journeys. But this was about 8 or 9 months ago now and i havent experienced it since. (hope it puts your mind at ease)
when's the last time you changed your fuel filter? I had a similar problem in an (shudder to say it....) Escort RS2000, I changed the fuel filter and it was fine. I'd let it red well into the red so it probably picked up all the sediment etc and blocked the filter a bit?