The problem is most likely caused by uneven set up of the springs. If you sit in traffic with the clutch lever pulled in the clutch plates should separate fully so that there is little or no friction between the plates. If the clutch isn't pulling off evenly there is 'drag' on the plates which causes friction and heat and warps the plates which causes more drag etc. As the plates warp they drag even more until there is so much drag you can't hold the bike back and you have to stall her on the brake. Because the plates are warped the clutch is all out of shape (like a stack of buckled 45 r.p.m. records) and the pressure plate is forced further out so you will feel that the clutch lever at the handlebar goes slack. When the bike cools down and the plates flatten out again the clutch lever resumes its normal position. This can seem weird until you realise what has happened. What you need to do is remove the primary cover and check that, when the clutch lever is pulled in, the pressure plate is spinning flat when spun by the kickstart. You'll probably see the pressure plate 'kicking out' when you swing the kickstart - remember to do all this with the clutch lever pulled in; you can even tape or bungee it to the handlebar. To get the pressure plate to spin evenly you'll need to tighten the spring nearest to the point where the pressure plate is kicking out. Most of us, being aware of the inadequacies of the A10 clutch, tend to engage neutral as we come to rest so that we are not heating the clutch up.