Hello all, as above, new to me K75C - my first K after years of airheads. It's only got 19,500 verifiable miles on it.

The problem shows itself as randomly losing all power above 4,000 rpm, there's no pattern I can see to it, sometimes it will only do it for a mile or two, other times for much longer - it's virtually unrideable when it happens. Large throttle openings cause it to completely bog down though it does keep running, using small throttle openings you can coax it up to about 40mph.
So far I've changed fuel hoses, fuel filter, airfilter, spark plugs, replaced the (split) crankcase breather, checked and set the TPS and, just now, measured the pressure at the exit of the fuel rail which shows 37psi but then, it's running fine at the moment.
All the plugs tend to look as though it's running rich and on one journey home when it was running badly it used a tank of fuel in 80 miles! The current tankful seems to be going at a more normal rate.
Any ideas would be most welcome?