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VNL670

First I would like to start by saying thanks to everyone that contributes to making this site a success from administrator and all the way down to the youngest members. This website is very beneficial and contains valuable info. I hope it stays around for a long time.
I am a newbie & I am an owner of an 07 Volvo VNL670 with a 565 HP ISX CM870. I would really appreciate any help on this following matter. The main problem is that when I'm bob-tailing or loaded, and I attempt to take off and start driving, the boost gauge goes up to 20-25 psi and then after a few seconds you hear a loud "pssshhhhht" or you get the whooooosshht sound (as if one of the inter-cooler boots came off or has a loose clamp or it's as if somebody just smashes your inter-cooler with a sledgehammer) and power just drops within seconds and boost gauge drops down to 5-7 psi on my boost gauge within 5 seconds. My EGT starts to creep up to reach 800 to 1000 degrees. I lay off the pedal or else the temperature will just keep on climbing. It will not do this at idling or while revving it up to around 2000 RPM. It has to be in gear and in motion to do all that and that's what makes it difficult to diagnose. I am getting around 3 to 4 MPG at best, which really sucks! All this happens with no derating of the engine and no check engine light. I have also noticed that my fan is always on and does not shut off (all A/C lines have been disconnected when replacing CAC and it is not the A/C line pressure switch). A friend mentioned to me that the line that feeds the fan is the same line that feeds the turbo air control valve for the turbo actuator and the fan may not be getting enough air pressure to disengage it due to some kind of an air leak. I don't know if it is related to my main issue.

I can not find the source of the whooshing sound. I have replaced the inter cooler with a Dura-Lite inter-cooler(best on the market). I have replaced the boots and clamps and pressure tested the system from the turbo outlet to the last boot on the intake, with no air leaks to be found. I thought it may be my turbo actuator, but that is also working fine and moves the way it is supposed to with no air leaks. I have ordered a DORMAN EGR Valve #904-5001 and should be here in a few days.
What can it be? Did anyone ever have any of these symptoms on their truck? pictures are available if needed. I am open to any suggestions or if there is an existing thread that addresses these issue please direct me to it. Any help is highly appreciated!!!