Same issue. Found an intermitten loss of crank sensor signal. Replaced crank and cam sensors, since they are close together. Machine would code and go into limp mode. Un hook battery cables off of battery, Then hook the 2 cables together for 2 or 3 min. Hook the battery back up and codes should be gone. Hope it helps.

cat had a similar issue with surging over idle speeds and it was due to the charging system overcharging. The higher voltages were affecting the sensors and other related components and causing the system to fluctuate trying to compensate for the voltage irregularities therefore creating the surge.

check the alternator output and make sure it is between 13.8v and 14.9v. Anything over that and there is an issue that needs to be addressed.
First check chassis grounding points and make sure they are good clean contact points. A volt drop test should be done between the battery and the ground point on the frame, any drop exceeding 0.5v indicates a problem and should be repaired.
Also check the ground to engine block in the same manner.
The overcharging will not necessarily trigger any codes so there may be none showing on the display and the MIL light may not light up either.

If no issues are found with grounds then replace the alternator.

good luck!

Might also want to check your fuel PSI,in the regulator at the time it surges and see if it is dropping pressure,could be a dirty screen in the LP Lockoff solenoid valve,or the regulator itself is gumming up with tar.

Possibly a dirty/ Faulty Mass air flow sensor.....

I had one doing this... with the key on and motor not running check to see if the check enging light is working. we had one and it really stumped us. found the check engine light was out...
just a thought