Experienced the same loss of power on my 1997 Vectra GT - Cat 3126 "a" vice "e", but could be the same type of problem.
Murphy was apparently alive an well on this "gotcha", since the loss of power was noticed pulling a CR-V toad eastbound on I-10 out of Mobile, enroute to Panama City. I do not recall hearing anything which I would attribute to the problem, but that's not say some "pop" did not occur. I couldn't find anyone in Panama City to look at it for the week I was there, ditto when I went to Destin. I even tried a couple of Freightliner repair shops on the way back to Colorado, but to no avail. One in Kansas had a flat policy to not even look at motorhomes, Freightliner chassis or not. It was a slow, dismal return.
The problem turned out to be a 1" - 1 1/4" split in a flexible connector joining the turbocharder output to the intercooler input - different vendors supplying different pieces, with a third party probably in there somewhere. Bottom line, the Cat-supplied turbocharger was pumping out full boost to the split seam right in the middle of a formed bend (of about 65 degrees) in the connector, resulting in the loss of enough of the boost to signficantly affect power to the engine. As with your description, the engine ran fine - no overtemp or other problem - except severe loss of power.
Functionally, the connecting piece in question also absorbs a certain degree of shock; given the turbo is engine-mounted, and the intercooler chassis-mounted. Point being, the split was probably as much due to eight years' normal wear and tear as any defect in the fabrication procss. The connector is quite flexible, appearing to be fabricated out of 3/4 -1" wide kevlar mesh impregnated with orange Permatex "Form-A-Gasket" material, all wrapped around (many, many turns) a form to produce the connective device. The turbo connection diameter was in the neighborhood of 1/2" smaller than the intercooler end. The entire piece would be charitably described, at best, as semi-rigid.
As I recall, the mechanic who correctly diagnosed the problem ordered the replacement part from Freightliner. He completed the entire R&R process by feel - the connector could not be visually sighted from either the top, bottom, or side! Maybe with the coach on a lift, but not flat on the ground.
Hope this helps.
