Serv, my Mack, also a 2003 CX613 (but with a 460 instead) has had a jumpy idle when hot since new. Dealership could not get it to not do it either, but told me some Macks just have this problem.
I've over 500k miles on it now and it still has a jumpy idle, and acts just like you described. I've also intermittant CEL that'll come and go, dealership has no clue why, thought grounds?? Mine will even shut down for a split second while going down the road, and then start right back up. Freaks me out when it happens while going through an intersection. Guages will bottom out and come back too, sometimes. Mine has done all this since new, hasn't gotton any worse, and I'm to the point now I just figure it's a Mack (and that's not in a good way)
I could go on and on about my dealings with Mack over this stuff and more, but won't.
You can check your codes with the cruise control switch's, but you must have a stored code (ie, CEL stays on) You do itby having key on, cruise power switch off, and hold RESUME/ACCEL switch on until CEL light goes out. The light will start flashing the codes. It'll be a two digit #, with a slight pause between seperate codes.
I have the list of codes, and can copy them for you, but it'll have to be tonight, gotta go make some money right now.