[Steve Davis]

This sort of thing can be very frustrating! A radio (or anything else for that mater) that will not work at all, is easier to troubleshoot than one that works, but just not quite right. 

Is the time from cold turn on until signal drop about the same each time or more random? If it is the same, I would guess a failing component. If random, it could be mechanical, possibly a riveted ground connection. You might use a clip lead connected to the chassis and touch the other end to every point under the chassis that should have a ground connection. Old style Philco tuning capacitors some times develop intermittent connections on the fixed plates, tighten all the fixed plate mounting screws. Recheck the alignment, does everything still peak in the same place or does the peak move one way or the other? I hade a 70 one time with a similar intermittent, it turned out to be a trimmer on an IF. There is a crimped brass eyelet that holds the thing together, the steel plate had rusted/corroded under the crimp, causing an intermittent connection.

Steve