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2004 Mack Ch613 limiting to 1300 rpm

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We have got a 2004 Mack Ch613 with an ac-427 engine in it. It has around 700,000 miles. Ever since late this spring the truck would run fine for about 15 or 20 ish minutes then it would go into a de-rate mode of sorts where it limits the rpm to around 1300. When accelerating it stutters around 1250 rpm. Sometimes after driving it for a little while longer it goes back to normal for a little bit then goes back funky. While it does this, the engine fan stays on, the powerleash engine brake does not work, and the cruise control does not work either. There is a code on the dash, it is 4-1 indicating a vehicle road speed fault. Our local Mack dealer says that is not the problem, at this point I am unsure where to go. Shutting the truck off for about 10 or 20 seconds resets it and you can drive for another 15 or 20 minutes before it acts up. Oil pressure appears to be good as well as coolant temperature, battery voltage shows right around 14 when running.

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Thank you very much for your reply. We took the 3 plugs out of the module on the left side of the engine, sprayed some contact cleaner in the plug and module and put some dielectric grease on the plug, stuck it back together and it has worked just fine ever since.

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