It really really sounds like it's running out of gas.
Here comes some questions, and Yea I know dumb questions, but sometimes the dumb questions jog the memory for something else.
-It runs the same in Prime, that rules out the Petcock vacuum line but doesn't rule out a dirty / varnished up petcock screen. Something to check since it's an older bike.
- Just replaced plug, Is it still clean? Is the Gap correct?
- Just cleaned carb, How dirty was it?
- Older bike, have you done a compression check?
- Are the valves adjusted correctly?