Always start with checking for strong spark, a bad condenser can put out strong spark when cold, but will quickly fade. Arcing across the points is a good indication of a bad condenser. Assuming that the ignition system is in tip-top shape, that sounds like a fuel related issue.
I would start with checking fuel flow, as well as the fuel level in the tank. With a gravity feed fuel system, a clogged gascap vent, or misrouted/to long of a fuel line are common culprits, especially as the fuel level gets lower in the tank. If the tank is venting properly, and fuel flowing well to the carburetor, I'd clean the carb.
Pay close attention to the small holes in the carburetor jets, just because you can see light through the hole does not mean it's not restricted by residue left from evaporating fuel causing a lean fuel mixture. Poking a small piece of soft copper wire through all the holes, followed by a good blast of air or carb cleaner works well. Do not use steel or anything to hard to clean out the jet, you may accidently enlarge the jet, changing the fuel mixture.
If the bike does not idle smoothly or takes a long time to settle back down to idle speed, and you've already cleaned the carburetor, you may have an airleak. Airleaks can be caused by a bad seal in the engine, or in the intake/carburetor joints. You can sometimes track down an airleak by spraying carburetor cleaner on a suspect sealing/joint area and listening closely to see if the engine RPM's change (sucking carb cleaner fumes into carburetor will also do this, so be careful when working near the carburetor).
Hope those suggestions help!