I've got the same problem on mine, and I haven't quite managed to figure it out yet. 11,800rpm corresponds to exactly 180km/hr in 6th gear - the CDI is supposed to have a lower rev limit in 6th to comply with the japanese 180km/hr restriction. However on mine, no matter what I do, even if I attempt the 'desestrict' mod, the bike hits the rev limit at 11,800rpm in all gears.
In essence, the CDI thinks the bike is in 6th gear all the time, even when it isn't. I even swapped for another OEM Suzuki 1991 VC-specific CDI and it does the same thing.
The only thing that got me past the 11,800rpm rev limit was buying an aftermarket 'derestrict' CDI box which ignores the rev limit in all gears, but unfortunately the timing curve in it gives me less power and makes the bike harder to start so I don't want to keep it in there.
Maybe i'll poke at it a bit more this weekend to see if I can figure out why, but I would start by looking at the wiring from the transmission to the CDI for the gear sensor.