My recently purchased '93 944T with 96K idles very rough, especially when cold, and every half-minute or so, it seems to briefly "shudder" and then even out again. This is not noticeable while the car is in gear. There is a significant amount of vibration during idle (this can really be felt in the driver's side door); it is less noticeable while in gear, but seems to still be present. The on-board tachometer shows idling at about 600K-800K rpm. I'm looking for a cause and a solution:

1) The Volvo mechanic that performed the pre-purchase inspection noted that the center muffler is weak (you can really tell this at highway speeds). Could the poor muffler performance affect this? Will replacing the exhaust (with the Volvo kit, I know) solve the idle problem?
2) Do I have a fuel system problem (dirty throttle body, injectors, etc.)?
3) It appears that there is about a 3/4 inch clearance between the oil pan and the front crossmember. Could the vibration be caused by poor engine mounts? Will fixing the idle problem solve the vibration?
4) The other problems identified by the mechanic were the breather box (I have tried to find it, I know where it is supposed to be but I just cannot see it behind the exhaust manifold, so I can't tell if there is an EGR in front of it) and the turbo oil return tube. I don't have any more detail on these problems. Could bad O-rings on these cause a vacuum problem leading to the idle shudder?

If it matters (this shows what a newbie I am), I have replaced the ATF and changed the oil since I purchased it 4 weeks ago. I have also replaced the oil cooling hoses that were identified as weak.

Sorry for the barrage of questions. I live in an area remote from a dealership and I am still deciding which local "Euro imports" mechanic I will be able to trust, so if there is anything I can do myself to solve the problem, I will give it a try. Thanks.
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Steve Van Steenhuyse ('95 944T, 95K)