Mate long time no speak
Taking into account you've ruled out thermostat and temp switch.
How old is the bike?
Check the following: -
Drain the system, how much coolant are you actually getting out?
What coolant WAS in it, colour?
What colour are the hose internals even the bypass hoses?
What's the coolant ratio in percentage?
Is there any sediment in what's was drained drained out is there any in the expansion tank?
It's very possible depending on age that someones drained the coolant and refilled with incorrect or not enough, or none at some point. This has induced corrosion/sediment and starting to block the rad thus reducing it's efficiency. Is the bike tuned or non standard in any way engine wise, chipped, got/had a power commander etc?
It's quite common especially with FZ1's people like to play!