I forgot to reply to the ECU reset.Disconnect the Positive battery lead leave it off for about 15mins then touch it off the negative lead for a few secs, now put it back on.There are two sensors for firing of plugs and injectors the Crankshaft sensor and the Camshaft or Cylinder Identification sensor. These going bad would be more of a misfire issue, stopping intermittently especially when hot or difficulty starting and/or rich running or stalling
A Throttle position sensor going faulty could cause the issue, but would be rare and it would most likely be surging when riding slowly. Removing it and turning it manually should show if it is the cause.
It could still be a stuck thermo wax unit as the little piston can seize. Bike that are idle for a long time or bike that are started occasionally and not allowed heat up fully when left idle for a long time can have corrosion build up on the little piston causing them to stick on or off. It could be stuck on the replacement throttle bodies also.You can probably remove it with out disconnecting the coolant hoses like the FZ1 or you could separate the unit, I am not overly familiar with the FZ8 but it should be similar to the FZ1. The piston should move in and out when cold but only about 5or 6mm. Since you have a spare throttle body you could practice on it first
A Throttle position sensor going faulty could cause the issue, but would be rare and it would most likely be surging when riding slowly. Removing it and turning it manually should show if it is the cause.
It could still be a stuck thermo wax unit as the little piston can seize. Bike that are idle for a long time or bike that are started occasionally and not allowed heat up fully when left idle for a long time can have corrosion build up on the little piston causing them to stick on or off. It could be stuck on the replacement throttle bodies also.You can probably remove it with out disconnecting the coolant hoses like the FZ1 or you could separate the unit, I am not overly familiar with the FZ8 but it should be similar to the FZ1. The piston should move in and out when cold but only about 5or 6mm. Since you have a spare throttle body you could practice on it first