"Might it be possible that a previous owner could have changed out the cartridge guide in an effort to utilize the more readily available 308 Win cartridge? Would it even be possible to shoot 308 ammo in this Savage 99 designed for 300 Savage (I understood the internal pressure of the 308 Win to be significantly higher than the 300 Savage cartridge)? If this were to be the case - could this be the cause of the feeding problem I experienced?"
Your rifle is built on the early receiver design. The .308 family of cartridges required a redesign of the internals of the receiver to allow for the longer cartridge. That didn't happen till the 900,000 serial and later.
I'm guessing the cartridge guide in your rifle is fine and I would not start there for a fix.
Try chambering the round with one continues action. Wind the carrier spring one notch and see if you tell a difference.
I don't recommend making multiple changes at one time.
