Greetings,
I ran into the same problem on my '93 854 a couple of months ago... It turned out to be the camshaft position sensor. To check your fault codes, primarily the camshaft position sensor ones, find the diagnostic module under the hood. It is located to the far left in the front of the engine compartment. With the ignition switched off, unclip the jumper wire from its holder and insert it into socket number 6 of the module. Turn the ignition on the the "II" position (before start) and head back around to the module. Press the button on module A for about a second, and take down any fault codes that are present. It will blink them at you in a morse code fashion. The ones to look out for in regard to the CPS are: 314: Camshaft position sensor signal absent
324: Camshaft position sensor signal absent intermittently

Also, check out Volvospeed for further clarification of the fault code reading. In addition, the Haynes manual for 850's is useful for looking up the codes as well as many other things you may run into. The sensor isn't too difficult to replace and will probably run about $150. Good luck,
Ryan
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'93 854 GLT, 152k