It is possible that the cam bearings were not installed properly. You may have a burr or nick in one of the bearing surfaces.
I don't want to Hijack your thread but here is something I observed firsthand:
Recently I witnessed the treardown of a real nice 302 that had been built by a very reputable fellow. The engine was in a 66 Mustang that saw a fair amount of open track usage in the SF Region. It had seen several hours of usage. One complaint conveyed to the shop doing the disassembly was irregular
oil pressure readings at times and valve lashes that kept changing. This had a lot of high dollar parts including one of the race specific (I think R302?) blocks. What was found is that the cam used by the prior builder was not intended to be used with this particular block. There were significant differences in the journal diameters and the ID's of the bearings. The cam could be "Rattled " around in the bearings from below with the crank removed. It was a huge oversight, fortunately there was not any major damage done.
Rick