I'm not all that familiar with the S4 model, but here are a couple of things that might be worth investigating.
1. Is it possible that a couple of the bolts that hold the compressor to the sump are loose (or unnecessary washers have been placed under the compressor/on the mounting pads)?
2. Is it possible that the idler pulley is not bolted tightly?
3. Can the positioning of the compressor and idler pulley be adjusted, even slightly, at their secure points? In other words, have you tried loosening the bolts that hold both of these items in place (with the belt off), and then moving them as far "out" as possible, and then re-tightening their bolts?
4. Is it possible that the idler pulley bracket is bent and thereby not providing the designed amount of tension on the serpentine belt?
5. Were different methods used to secure the AC compressor in the different years/models, and some PO installed an earlier version mounting setup on your car, i.e., replaced the motor (which changed the compressor mounting setup)?
6. I know you checked the diameter of the idler pulley, but did you check the diameter of the compressor clutch pulley? Many aftermarket or rebuild compressors are available that will "work" on these cars but do they all have the same sized clutch pulleys? What if your PO replaced the compressor or clutch pulley and the diameter of the replacement clutch pulley was incorrect?
I dunno, perhaps I'm grasping at straws here, but I'm following this thread with interest, and hope you figure it out.
'88 Quad - "Claudia"