Rev limiter cutting out early

I've got a 1998 model SV - owned it from new - it's only done 27,000 km. I use it on track days regularly and never had any problems until last weekend. After 3 sessions (rev limiter coming in at around 11,000 RPM) I noticed it started to cut out at 9,500 RPM then at 6,000 RPM which made it unridable on the track. Runs perfectly OK on the street, goes hard until you reach 6,000 RPM and then acts as if you hit the kill switch. I removed and drained the fuel tank, checked the in-tank fuel filter, the vaccuum operated fuel tap, the pulse diaphram. All are working fine and are pristinely clean, so I ruled out a fuel problem. I have no way of checking the digital igniter unit except for testing all the wiring to and from it. I checked the throtle position sensor which works fine. QUESTION - has anybody come across the problem of the igniter unit cutting out early?????