the check engine light has come on several times, each time the code reads po171 (running lean). the car will often lose power and sometimes almost kill when at a stop. i had the injectors and rings motor-vac'd and it ran great. the problem returned shortly after with a code saying that the problem is undetermined. anyone have a similar story/solution? thanks.

I think a leaky or disconnected vacuum line will cause a lean
condition (as it introduces more air). The electronics will try
to compensate by injecting more fuel but once the leak is large
enough it can't be compenstaed for (and the CEL light goes on).
I'd try a visual inspection of vacuum lines. Bay 13 has
some guidelines on this:

http://volvospeed.com/Repair/vacuum_lines.htm