Those symptoms can point to a lot of things.

Lots of times, when problems come and go, the carb has crud in it. I think K&B carbs have a space inside them where trash or gunk can get trapped. Find someone with K&B experience if other things don't work and see if he'll open it up for you.

Older engines that were run on 100% castor for the oil content would varnish up inside. After a season or two, they'd start to act up like yours. The de-varnishing routine was well known to guys who flew a lot. You'd basically strip it down and remove the varnish. We didn't know about the crockpot back then, and that just might work for a varnished engine. It'd be a heck of a lot easier.