It would be useful to know whether you were accelerating, at a constant speed, or on a closed throttle?

Most likely cause is the back of the bike sitting too low, or conversely the front to high. This might be down to incorrect or imbalanced sag front/rear, being lowered with longer links but not having the front end altered to match, or a saggy shock and/or poor damping.

Mine had a tendency to do the sort of thing you describe when the OE Sachs shock began to fail (from 10k miles onward).

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