Why is it my kx450r starts and idles fine but back fires and misses ?

hi everyone need help have a 1999 Yamaha sx 700 triple..been having a problem with a miss fire this season.. carbs all cleaned in November.. initial start up is fine. idles mid throttol and wide open..problem is when I get it up to temp..say running 50 mph and come to a stop.. rail road or 4 way stop sign..i get a miss fire or down to 1 cylinder or 2..go to give it throttol and nothing bogs no gain in rpms.. if I let it idle it hit and misses on 1-2 cylinders..will do this till I shut it down...I fire it back up and it runs fine.like nothing happen.?????? did this to me every time I stopped heading to a friends house about 30 miles..did it ones at his house..and rode the rest of the night and home with no problem..
thinking cdi box or or smooth capasitor? any thoughts on this? thanks in advance

Check valve lash

Could be a loose connection at the plug or at the coils. Look at in the dark and see if you see any spark off.

Check the battery cables. I had the same issues with my 96 tacoma, The battery cable connection at the battery can be cranked down but still be loose. Hard to explain, doesn't make sense, but just check it out.

You are equipt with a ignition control module,located near the A/C pump when this fails the vehicle stays running and wont accelerate, but will stay at idle,

First, if you increase the rpm's while the vehicle is idling, do they come up normally or does it miss then too? I would check to see not only if you have spark but what the spark intensity is. (under load, a weak spark can actually be extinguished) firing through the exhaust means that either you have a burned exhaust valve , bad exhaust lobe on the cam or that the ignition timing is so far off that it is firing when the exhaust valves are open.If you have a 4.0 engine, it is possible that the outer ring on the crank pulley has turned and what you think is correct timing may be not even close.

Sounds like you have a leaking cylinder head gasket

if this a three five v 6 tha timing belt could be off one tooth you can also youse a cheap timing light to check each plug wire at a time look for no fire or erratic spark

could be bad glow plugs or plug heaters, faulty wiring to the coil, vacuum leak.