I have noticed on occasion that the LNAV guidance gets out of whack so to speak and the aircraft starts chasing back and forth to catch it. When I see it happen it is when attempting to turn at a waypoint. The aircraft for some reason will only turn 5 degrees even though the MCP is set to AUTO for rate of turn. The plane will not maintain lateral guidance and start chasing back and forth to reacquire. This is not the old "s-turn" issue. When sitting at the computer and watching it happen, I will see it miss the turn due to only banking 5 degrees or so and then it starts going +30 degree rolls going close to 45 degree banks to try and reacquire. I have to take it off LNAV, go to HDG SELECT and take it off AUTO and put it on 25 degree bank to get it to sort itself out. Then after fixing it and selecting LNAV I may not have the problem the rest of the flight. As in, I will put it back on AUTO for bank mode and LNAV and it will make proper turns up to 25 degrees the rest of the flight without issue. Has anyone else seen this? Going to put in a ticket on it just the same.

Rate of turn knob on the MCP has nothing to do with LNAV.

It just works with TRK/HDG selection.

 

Can't really help with your problem, though. Might be sw issues

Search AVSIM (or Google) for "PMDG S-Turns." What you find will have you fixed right up.