Need help with a diagnosis

[Mister Furious]

Hi all, I could use some help diagnosing what’s going on with my 05 ISX; it’s in an 05 T600. Full rebuild 570k ago; was de-mandated and the VGT replaced with a regular turbo at that time. ECM tune was done by {Horrible delete tuning shop name deleted by Rawze} and set at 600hp. This is info from the previous owner. I’ve owned it for 3 months and put 30k on it.

A few weeks ago it started to surge at idle, for a few seconds. Gradually it increased over the next few weeks until now it is constant. When I start the truck it will idle perfectly for about 30 seconds, then it’s like a switch gets flipped and the surging starts. Doesn’t matter, hot or cold or anywhere in between. I can turn it off, restart it and the cycle repeats. I swapped the timing and metering actuators off of a perfectly running truck with no change. It sounds like it’s got a hot cam in it. Apply the throttle and it runs perfectly. Mileage has remained the same; 6.1.

Yesterday it suddenly would not build rpm’s past 1650, in or out of gear. At the same time engaging the cruise would cause the throttle pedal to go dead. Turn the cruise off and instantly the throttle returns. Is my ECM on its way out?