Check this link if it's not already done.
http://v8sho.com/SHO/MechanicalCheckList.htm
1) crank position sensor
2) oil cooler (just up the oil filter)
3) transmission (try it cold and hot at low and high rpms)
4) cam position sensor
5) parking brake adjuster (it's not really an issue and simple to resolve)
6) tire wear (can be a symptom of a much complex problem : bad springs or SARC struts which are impossible to find new)
7) CD changer (in the trunk, they will skip or just not finding the cd, these prob can be easely repaired)
By the the time, I found some other weak points and you don't have to be a Gen 3 SHO Fan to find them :
8) spark plug coils (watch for misfire when trying it, rear ones are a good job to replace, watch for at low rpms and high rpms espacially)
9) subframe bushings (front ones are weak and at 136k, they will probably almost scrap, you can replace front ones by rear ones which are tougher, if you hear something knocking in the front when rolling, it's probably one of them)
10) struts and spings (unique to SHOs, very difficult to find, check them with the ignition on, best way to find replacement is to watch for a recent lower mileage scrapped one and take his shocks)*
*About strut going bad I also heard a story from an old time owner that a strut can go bad because the height sensor was wrong (bad or unsnapped not sure) and sending false informations to the computer. This owner and his mechanic find the prob after the 2 or 3 strut replacement...
Good luck,