The fact it happens occasionally sounds to me like an electrical problem, and the fact 2 gauges are affected is a giveaway (if it was a failed speedo sensor, why would it affect your rev counter?)
The fact it's intermittent makes it hard work to track down (temperature / humidity / mechanical movement and all sorts of factors can have an influence), but it can be done.
As silly as it may sound, does it happens with steering action? ie, could a wire be getting trapped / pulled with the bars? Or could a chafed wire be coming into contact with the forks as they move? (Plugs / sockets may be clean and tight but its worthless if the wiring is damaged in between. Bear in mind manufacturers can be a bit tight with wire lengths (cost) and don't always leave much "slack" on wiring looms.
If the wiring to the dash is ok, then its a case of following the wiring back to the ECU and any sensors (throttle position sensor and speed sensor wiring esp, on the later FZ6 there is wiring relating to both sensors), (I'm not familiar with the setup on the FZS bikes and haven't got diagrams for it either) and making sure they're clean and tight as well (bear in mind engine-mounted sensors and wiring get exposed to temperature fluctuations).
My guess is you've got either a loose wire (vibration / heat), or corrosion (traffic film from the road) on an earth point somewhere (earth points are almost always the weak link in the chain).
IMO a faulty sensor is unlikely (see above), and a battery / fuse fault is unlikely (different symptoms).