Saab 9-5 misfires

My wife has a 2000 saab 9-5 which is misfiring. Our mechanic changed plugs, fuel filter etc to no effect. He says its misfiring in different cylinders but Saab doesn’t put out enough useful information to the compute to diagnose further.

This may be irrelevant but we just had a problem with the ABS system which required replacing the ABS module ($$$). Our mechanic thinks we may have a problem in the electrical harness and suggests taking to a Saab dealer.

After dumping a lot of money into the car recently I am getting worried. Any ideas?

compresion test,inj drop test,cyl leakdown test ,basics old school. eg: is it a valve ?is it an injector any codes or bullitons?step one