Volvo: 86 740 T sputters at boost, coolant leak, vacuum line

Question
I've had 5 volvos, now on my 2nd 86 740T. Do most of my own but go by the addage "if it ain't broke don't fix it".  My cars go to the junk when I'm done.  Would like to keep this one going.  Changed the head gasket on purchase to solve coolant leak.  Engine good overall.  Missed an oilchange and heard turbo noise increase.  Sudden change in performance, now engine sputters and cannot run above 3500rpm or at all with throttle to boost pressure.  Dropped exhaust and tested for blockage.  Cat is good.  Miss-fires the same, but can hear the cat running smoothly.  I expect fuel supply problems or waste gate openning but do not know how to proceed. Thanks for your concideration
Gary.

Answer
Gary,
Poor preformance, bad turbo, leaking intercooler or hoses, ignition system, fuel supply or pressure too low. Test the car with a fuel gauge and see what pressures you have. Pull off the vacuum line on the fuel press regulator and see if the pressure increases as it should. If the regulator leaks it is not any good. Test the feed pump make sure it works. Take a compression test to see if you may have a weak cylinder or bad valve. Maybe a broken valve spring.
If the wastegate is stuck open it will also have limited power. Wastegate should be set at about 7 psi. Some of the older cars have some wire harness problems, look to make sure the harness is in good condition.