no power

[JD]

Please help me figure out what to test next.  My 87 5000cstq was running
great- then a slight miss at low rpm high load -then terrible misses and
no power at all.   When throttle wide open no boost and no power and a
violent vibration . I have done the following and have not resolved
problem-  compression check-100 psi at 8000 ft .  no vacuum leaks - took
all hoses and  intercooler off and checked for leaks,  changed rotor and
cap- all cylinders have spark at least thru wires as shown by timing
light on each one.  cam timing mark at proper point when tdc.
Took front cat off and no evidence of plugged nor melted- I am assuming
downstream not restricted,  turbo does not have excessive play and does
spin free by hand.  fault code 4444 only-(no faults)  What am I
missing?????  Thanks---- John Dyer