Bike stalls - but was running well

Hi guys, 

I have a 1987 GPz750 Unitrack, I think an A2 with full fairings. I have had it for over a year, the previous owner did a complete restoration.  It runs well and pulls like a train, of late, it is stalling when hot.  I do understand the vapour lock situation and am paying attention to that, but before I pull the carbs and go through them, I wanted to see what other things I might try looking at.  On my way home today, it was stalling so I switched it off because I was stuck at roadworks and I didn't want it to overheat. When we started to move, I hit the button and it wouldn't start, I treated it like it was flooded and used wide open throttle and it fired up. It was still hesitant to run until I got to the 100kph speed zone and I gave it some beans to see what would happen and it pigged and farted.  It eventually evened out and ran well the rest of the way home.  I have been having problems with the fuel cap not sealing, it will leak fuel if it sits on the side stand.... just being a PITA. It had a brand new fuel tank fitted when it was restored and it won't seal, so I can't imagine its a blockage of the tank vent. I can't smell any fuel like caby leaks, in saying that, I have a feeling that when it  stalls, it's running on 3 cylinders.  I have checked to compression, which is excellent with no more than 5PSI between them, I have tried looking for vacuum leaks and can't find any leaks. I had a look at the plugs, they seem a bit on the lean side, so I figured that if it was a faulty coil, that I would loose two cylinders, not just one. It's a bit of a weird one, so I thought that I would ask the brains trust.

Noel