JD, Very impressive thing to check as a concept. Not so infrequent a plug is mis-wired. Well worth verifying while the plug is open under preliminary inspection. Also tug on the conductors inside the plug to verify they have not been cut or broken both under the terminal and at the strain relief which would cause a intermittent contact issue.
A lamp that is fitting loosely within the lamp base, not holding by way of it’s pins or pressure fitting is a lamp in a bad lamp base. Doesn’t always matter if it’s charred, a lamp not supported inside the lamp base is a no good lamp base. And no, it’s not warranty either. This especially after FEL’s potentially have been in it.
Most likely if the lamp is loose within the lamp base, that’s the cause of your problem. In some positions the lamp has good or loosely good contact, in others it won’t. This by way of focusing or bumping the lamp or even expansion & contraction of metals could cause flickers
etc. Potentially the 120v could be enough power to arc in working thru a bad contact.
To the best of my knowledge
Altman has not recognized the C3A for use in their fixtures. Could be wrong in having time to have tested it & perhaps now saying it’s okay to use. While a often recommended lamp base for use in the fixture, check earlier postings on this lamp base for the why & the ones Altman does specify.