I have had this problem since day one with the Van aaken box. When I am towing, at arround 60 MPH 1900 RPM the engines sometimes starts to sound almoste as if it is missing. Especially when under a load and only when towing heavy . Give it more gas or back off and it smooths out. I have been operating with this behavior for the last few years and have always hoped that the next modification would cure it. There is no indication of a problem on the gauges. RPM steady, fuel presure and EGTs also do not vary. Boost steady although the needle does vibrate at anything over 22lbs in overdrive. In third it is steady up to over 30 lbs. I do not think the boost gauge is related to the problem.
Ron
Ron