I have a 2004 Volvo with a 435 ISX. The problem that I’m having is this;
Driving along with or without a load I step on the pedal and get 35psi boost and without warning boost drops down to 0psi. No check engine light or anything. No black smoke out of the stacks.
I could lose boost for 5 sec or 5 min. Then all of a sudden full boost. Then I could drive the truck for an entire day without any issues and then it starts all over again. Friday I was driving and boost was all over the place. Under heavy pedal I got 35psi then within a second dropped down to 0Psi and instantly back to 35psi.

Drove the truck to Kenworth Cummimns dealer. They put the truck on the computer, got billed $129 and were told there is nothing wrong with the unit. However if the problem persists to bring it back.
Having said that they recommended that I take the turbo off and look at the blades to make sure it’s free spinning also they asked me to install a new air filter located on left side of engine, the one that supplies air to the waste gate.

Another driver is doing a run with the truck he had no problems yesterday, however today same thing 35psi – 0psi -35psi – 0psi – 35psi – 0psi and remained there for a few miles. The driver started to pull on small hill, 0psi barley made it up. He revved the engine up to 2000Rpm and got full boost instantly.

Any advice no matter how silly it might be would be appreciated.

Update:

The unit was at cummins and they found out that the exhaust manifold pressure sensor was faulty.

We have put on about 6,000 miles since the fix and no problems.