|I cannot view the schematic on my computer. However, judging from similar circuits, bad fixed mica capacitors in the FM section of the IF transformers COULD affect the FM without affecting the AM. If they were to become intermittantly open, their capacitances are so small as to probably not affect the AM portion of the IF transformer much, but they will greatly affect the FM portion. This isn't necessarily your problem, but it is a possibility.
Other possibilities would be components that are particular to FM-only sections of the radio; namely capacitors. If the radio has a separate FM front end, trouble could lie there, and it could also lie in the discriminator circuit.
It'll be easiest to tell once Edd finds a schematic, but I wouldn't rule out IF transformers. The impedances of the FM portions, which are supposedly in series with the AM portions (from what I've read), are low enough to almost completely bypass the very small parallel capacitances at the AM IF frequency.