fzs600s unrideable under load

[foccerjohn]

Hi,

03 fzs600s now just over 100k, runs perfectly and revs crisply at a standstill, but under load has become unrideable. This problem was at about 7,000rpm, but has got worse, so now at lower rpm.

My dad thinks it could be the ecu generating a weak spark, any ideas?

Thanks,