IS250 RPM Fluctuating 600 to 900

[Darwinite]

Hi folks, new member here, hope you can help me. I have a 2010 Lexus IS250 (Automatic) with 177,000klm on the clock.  Recently my OEX alternator failed and was replaced (under warranty) with another OEX alternator. Since then the rpm fluctuates when the car is idling and is in drive or reverse, not in Park or Neutral, so when under load. This only happens after driving the car for about ten minutes or more. The rpm fluctuates up and down from around 600 to 900 constantly and also when this is happening the alternator shows the output charge voltage also fluctuates from around 12.90v then up to 13.95v, when idling smooth (Park or Neutral) the voltage output is 14.10v. l have recently discovered that if l have my parking lights or headlights on the problem with the rpm fluctuations stops, so now l drive around with my parking lights on all the time. l have recently had a diagnostic analasis done and no faults show up, this was done by "Darwin Diagnostic and Performance" (l am having the ECU remapped but can`t until this problem is solved). John from DDP is thinking that maybe a VVT solenoid is faulty so the next diagnostic session he will checking them (l do believe there is 6 of them?) One more thing to mention is all the sparkplugs have been replaced with OEM ones from Toyota. Given that most of the front of the engine has been removed and put back together again (for the alternator replacement) and then the top (for the spark plug replacement) maybe there is a faulty electrical connection somewhere? This problem started when l had the alternator replaced the second time.
Has anyone had this similar problem, any ideas anyone?

Regards Vince