engine stalling

On a recent trail ride in the peaks the engine on my drz 400s has been cutting out. The problem seems to be if the bike is revved and then the clutch engaged. It then jsut loses revs and stalls. It seems to be worse going down hill although i have had it happen on the road as well when changing down the box then engaging the clutch and as i blip the throttle it just dies.

Anyone got any ideas where i could start my investigations?

Cheers

So it happens when you blip/power accel away from riding?

But it doesn't happen when stopped and power away?

Yeah, only happens when already on the move. If i blip the power say when changng down the box and at the sametime engage the clutch, the engine just cuts.

Anyone with any thoughts...please????