600SDI goes to 1 cylinder @ 6200rpm

[SummitInn]

Coopers 09 600 SDI runs great at first pull. Idle a little low but not the problem.. After riding 20-30mins at 6200 down the trail or breaking trail, it goes over to 1 cylinder. playing with throttle it will start back on two. Pull the throttle wide open and the machine is back on two and runs great. As soon as you start to let off, it goes back to 1 cylinder in and out.....?

It's not realy a Bog, more cutting in and out.

Have changed plugs and relays, before I tie into it, I could use some Ideas..

Thanks