Hitting a invisible brick wall

[darrsi]

Thinking about it - this sounds exactly the same as what mine was doing when the spark plug caps were breaking up  (although mine was happenening at around 80 odd. Might be worth replacing them (there's a red NGK set people use that work perfectly) - not very expensive from what I remember?

http://www.ebay.co.uk/itm/1x-NGK-Resisto...541a3dc4f6