Yamaha FZS 600 Faser - Losing a cylinder when hot

I recently bought a 1999 Yamaha FZS 600 Fazer. It was absolutely fine on the test ride. Did a couple of other short rides (about 20 mins), also fine. Then today after about 15 mins I lose a cylinder. It feels like it just stopped firing.

I got to my destination and left it for a little while. Then after 10-15 mins started the bike. It felt fine. Then after a min or so the cylinder goes again and I’m back to riding a triple (or a twin, but I’m pretty sure it’s just one that went). It feels like it comes back when revving really high, but splutters lower in the rev range.

It was serviced before I purchased it, so I’m assuming the spark plugs are fine.

I have a few questions:

1. Does anyone have any ideas on what might be wrong, or suggestions
   on what I could check?
2. How do I find out which cylinder isn’t firing?
3. The previous owner fitted a scottoiler which they removed before selling. Looks like the vacuum spigot is on the throttle body. Could that cause a problem if it wasn’t replaced properly?