Simon, I have this sometimes aswell - usually if I've been poodle-ing, (and I mean poodle-ing like not going over 2000rpm) around all day and then suddenly decide to give it some welly. It'll like pick up, then falter for a split second and then take off like a scalded cat! (This generally in 1st gear only).
But if I have been driving around not so sedately it doesn't do it, never!
I've come to the conclusion, (and this is my way of putting it) is that the engine has dozed off and giving it some unexpectedly makes it go into shock - but thereafter, once awake, it's fine....lol.
ps. Both VIS are OK...