Lee, i've got the exact same problem as you, mine only does it also when its warm, it will miss inbetween 2800rpm - 3200rpm, and also the rev counter goes haywire sometimes bouncing all over the place. and basically run like a bag of nails - though the problem is v.intermittent
i couldnt understand why it only happened when it was warm, until i was told the hall sender is more likely to fail when warm!
i've checked & cleaned everything, so i ordered a new hall sender of eurocarparts for ?30 and im gonna fit it this weekend.
ill let you know how i get on.
Craig