T-5 boost cut/rough idle
 

I have a 97 850 T-5 sedan with 170k mi that is basically stock except for a cold air intake. Under hard acceleration it pulls hard as soon as I get into the white on the boost gauge, but when the gauge gets about 1/3 of the way into the white the car seems to lose all torque. IT feels like it runs rough in the upper rpms. After it did this a few times it started idling rough when in drive (but not in park). I just changed the plugs and they looked good. The wires also look new, but I don't know about the cap and coil.

I have an excellent Volvo mechanic here (Foreign Carriage if you are in Thousand Oaks, CA) that I'm going to take it to, I just like to be as informed as possible before taking it in.

Thanks for any help/suggestions you guys can provide.

I forgot to mention I have changed 2 of the 3 vacuum lines coming off the turbo solenoid. The one going to the wastegate and the other that connects to the air inlet. I can't find the destination of the 3rd line so I haven't changed that yet.

Sounds like you are misfiring. If the wires are OEM ones they will have a date stamped on them. See what the date is if it has it. The cap might be bad as well.

Them induction kits can soak up heat and cause rought idling as well.
Faulty boost control valve or dizzy cap could be the problem as above says.