1999 Volvo VNl610 with a Detroit 12.7

I'd be looking @ alot if things before an inframe. Because the problem is NOT a constant one you might want to have the "waste gate"of the turbo looked at to see if it is sticking and not allowing the turbo charger to work properly.

 

Take a oil sample to the lab that will tell you the condition of the engine

 

Injector, maybe

 

There's so many possibilites. One I can think of is the TPS (throttle position sensor).

 

I had that same truck I kick my self in the a ss everytime I think about it, my truck on ifta was 7.2-7.4 when I lost a injector it dropped down to 6.2 and was just week

 

The 12.7 detroit is by far one of the best engines ever built and verry simple I'd forget comuter or inframe for now and start with sonething a little simpler.your symptom would lead me towards air charge cooler so check your hoses for cracks pinholes or loose clamps first those are easy fixes you can handle yourself.if those r good have a mec test cooler