Help needed with sb 318

H Monster, if I was in Missouri, I would help you out. From Joplin, MO area originally.
Here is my 2 cents.
Starts up OK-Check
Runs for a little bit well-Check
Starts misfiring after short warm up drive-Check
Sounds like your condenser is breaking down. If it is points that is. Had this problem before on a couple of cars. Replace it and enjoy the ride.
On a electronic, the pickup may be causing the same issue. Primarily if the shaft is worn and wobbling around. The vacuum advance pulls on the pickup plate and can shift it out of position, causing a poor signal. Disconnect the vacuum advance and road test.