Okay, well going by what you're telling me, if we assume the CDI system is not at fault, then that leaves you with the ignition coil, spark plug and lead.
I really would just put a brand new OEM ignition coil on there as I am concerned you've fitted a Chinese one on there.
I fell into this trap when I did the rebuild on my DTR and I bought a 'Racing Coil' and it worked fine for like 15 minutes and then would be saturated and not able to maintain spark.
I had just rebuilt the bike and everything was new on it and it took me until my brother bought his RD350 with the same problem and the same Chinese coils for me to tweak. Bought brand new OEM coil and the problem went away.
Another common problem that sounds like yours is having water in the exhaust expansion chamber.
Make sure the inlet and exhaust are completely clear of any debris
You can eliminate the airbox to rule this side out, if it's only for a short while and you don't live in an overly dusty region then you'll be fine.
And give your exhaust a thoroughly good clean to decoke and remove any particulates there
If you have the stock RE exhaust, these have a honeycomb catalyst inside them which must be coming to their end of life by now, so an aftermarket DEP pipe would cure that
