Need help - steering wheel wobble

[JoeFromPA]

Hi all,

I've got a randomly-appearing steering wheel wobble at speed that I don't want to throw money at to track down. I consider myself fairly good at diagnosing these things, but this one has got me stumped and I'm going to start trying things soon.

99 Boxster, 61k miles, about 30k on the 18" turbo twists sitting on there now. Shod in michelin pilot sports (not PS2s). Tires only have about 8k miles on them. Suspension was re-done less than 10k ago with bilstein HDs on stock springs. Alignment was performed at that time and returned to spec. Car went over something (don't know what, was my father's at the time) that slammed into the pass-side front jacking point pretty hard and messed it up, but no other visible damage so I don't see how it could've thrown anything off.

My steering wheel shimmies and wobbles at a variety of speeds. But those speeds are rarely consistent. For the past few weeks, 70-74mph has been very safe...today it wobbled badly at those speeds and went away at 78-80mph. Sometimes wobbles down around 50mph. Usually safe from 0-40 and 56-65mph from all notable shimmy or wobble.

On the same day/drive, it'll sometimes wobble at a specific speed and then stop when I return to that speed 5 minutes later.

I want to eliminate as much as possible before going and getting the tires re-balanced - so I'm looking for tips here. The wheels look great in terms of round-ness, no obvious bends. Tires are in great shape and are michelins.

I've inspected the front suspension and steering BUSHINGs and all look good - actually, they look really great for a 12/13 year old car.

What else should I inspect or examine on this car before dropping $100-150 to get a quality re-balancing done?

Any and all advice welcome.

Joe