"funny" steering feel only on serious downhill (road)
 

So, it started happening a while ago but, when going downhill at a pretty good grade and longish distance, it feels like my bike is not tracking straight. I've checked the rigidity of all the suspect parts (bars, stem, headset, frame, wheels, seat post, bottom bracket) and everything seems solid and tight. I don't have trouble shifting (thinking alignment of the frame). The wheelset is about fourteen years old but, solid as a rock and I had them repacked recently along with new cones in the front. I'm at a loss. People riding behind me don't notice anything but, they're also descending and are more focused on what they're doing. Short of pulling everything apart and inspecting piece by piece throughout the bike, I have no idea what to look at. Any suggestions would be very appreciated. Feel free ask questions for clarification.
Thanks!

Slightly bent fork or the front wheel is seated in the frame to one side (not perpendicular to the ground.) It won't be easy to see in either case--if you try riding handless and get tracked off to one side you'll know it's one of these.

Think on it for a minute and you should be able to discern the principals that govern the direction of the pull and the irregularity in the fork.

AFAIK a DIY approach to checking fork alignment is a tough one. Best bet might be having it checked on a jig.

But as far as the frame goes, there's the string test for a start... and eye up the head tube against the seat tube (can be a bit hard these days with flared head tubes, but usually the diameter is the same at each end).

Check your wheels for dish by flipping them in the frame, but not before sitting the bike upside down, releasing the QRs and rotating the spindles to ensure they're straight.

Actually once you have confirmed that the rear triangle is centered using the "string test," and wheels are dished and true, with both wheels sitting in the middle of the stays/blades, it's not hard to visually check fork alignment. Turn the bike upside down - best done on a rotating stand, otherwise use a friend to assist. Sight over the top of the front wheel toward the back of the bike, noting whether it is tilted in relation to the rear. Then sight along both sides to see if one side of the rear wheel shows more than the other, making sure of course that you hold the wheel straight. Finally sight along the side of the front wheel again toward the top and seat tubes, again checking to see if the appearance is different from side to side.

Park used to make a gauge for checking fork alignment (the "FT-4"):

http://www.os2.dhs.org/~john/park-ft4.jpg

Perhaps your LBS or co-op has one?

Be sure that your headset is not adjusted too tightly. If the fork isn't free enough to self-center itself, then it might track funny. You should also notice the problem after a fast turn, when the bike wants to keep turning and not return to a straight-ahead position.

It's also possible to cut cable housings to the wrong length and create a situation where a housing tries to push or pull on the bars.

There is a good and cheap method to check rear frame alignment. Take a piece of string and tie to one side of the rear dropout. Loop the string around the headset and tie to the other side of the the rear dropout in the same place. Pull the string tight. The measure the distance to the string from each side of the seat tube. If that measurement is equal, as others say it may be a fork problem.

BTW I would always check this frame alignment on any used bike that I might buy.

No one asked or mentioned, but does the bike have a suspension fork? If so, sometimes the fork compresses unevenly, causing the wheel to deflect to either side.

Thanks for all of the great suggestions for things to look at. I will start going through these to see if anything shows up.

Btw, it's not a suspension fork. Ti road frame w/ carbon fork and steerer.

Again, thanks everyone for the ideas.

Yeah, that's another possibility; I collected a bike to ride back to the workshop last week that had super-spooky handling... I was thinking something must be seriously wrong so I stopped and checked it out.

To my surprise it was just the result of the most badly indexed headset I've come across. Upon disassembly one of the cartridges felt a bit damaged, but lacking an immediate replacement was reused; thankfully it was quite passable after being properly reassembled.

Obviously some total goose had assumed the top cap was for fastening rather than adjustment...

I haven't commented, and the others have covered some bases,

BUT

What, exactly do you mean by "it feels like my bike is not tracking straight"?

Is it pulling to one side? If so is it always to the same side? Do you have high speed shimmy? Are you overcompensating when steering? or what EXACTLY?.

Diagnosing a problem accurately (especially blind) requires a good description of what the problem is. Absent that folks are just throwing shist against the wall to see what sticks.