Carb slide/piston jamming

[Steam]

I think one of my carb slides/pistons is jamming.
I get up to about 6000rpm, then it feels like a carburettor piston jams, and then frees again. There's a flat-spot in power at that point, but then mostly it frees up again and the power surges back. ZOOM, back up to 150kph! Most enjoyable.

I thought it was the stator for a while but now I'm fairly sure it's one of the carb slides. (thanks to Limbimtimwim for suggesting the problem).
I had the carburettors cleaned at Motomart in Lower Hutt but it hasn't made a difference.

Should I take it apart and inspect it myself? What should I be looking for?

[What?]

Sounds like the rubber diaphragm in the top might be perished.