[Fot] What causes the hopping?

[sherry robyn]

With my TR6 on a sweeping turn, if I'm going fast enough that the car should
be 
sliding (where any more would cause it to come around aka spinout), it
will 
invariably start a sort of hopping along sideways.  These are very short
hops 
almost like a very severe vibration.  When this starts, it is not easy
to break 
out of it so it is a big time waster.

Does anyone have any idea
exactly what this hopping is, what causes it, or what 
fixes it?

Thanks for
any thoughts on this.  Jim