a flashing coil light does mean an engine fault
and I've known to get transmission fault messages when you're down on power as the transmission control isn't seeing the torque its expecting to see.
I'd first check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
Also check that the vacuum hose running to the dump /or turbo bypass valve is intact with no leaks as if the dump valve is opening prematurely you'll be losing all your boost
If this is OK then remove the high pressure intake hose after the turbo and have a feel of the turbine wheel - there should be no play radially at all - but a little bit of axial / end to end float is normal
The dump valve is on the turbo boost air (post compressor) side its there to blow off or 'dump' the excess boost pressure you get when you close the throttle ( like just before a gear change)
the important thing with a dump valve is that it seals properly when its not being operated if the valve has a torn seal or a broken spring then it'll blow off before it should and you'll lose boost pressure
this video shows how you can test it
if this is all OK then I'd say the issue is with the turbo actuator
they can burn out so its worth popping the lid off the actuator and checking inside
I've attached an image of a faulty actuator and you'll see the solder joints are burned
sometimes they can be brought back to life by resoldering the joints