I finally got my mk2 on the road!

, drives really well around town. But when I got to the fast stuff I get to just below 90, and it starts to kangaroo. it only does this at high speeds (when i really dont want it to be happening!)

the same revs at lower speeds are ok, in fact i'm really suprised at just how smooth the engine is.
I've looked through the other posts on here, but in those threads it seems revs dependant, not speed.
I'll check things like the leads, plugs, dizzy, air filter and breather hoses at the weekend, anything else you guys can suggest?
Also between 70 and 85 it really didnt feel very stable, but it might just be me comparing it to my Vectra which would obviously be more stable.