Weird vibration in car at higher speeds

[Secret Chimp]

I just replaced my left inner ball joint and have new aluminum wheels on new tires, but I still have this strange vibration at highway speeds. What's strange about it is that it isn't consistent. I can get a shake in my wheel at 70 or 65, but it can just kind of go away at some point. It's a side-to-side vibration, not a kind of one-edged jerk or a less regular junky feeling like something is about to fall off. It seems to go away when I pass over an expansion joint, though on my way home today I tried putting the car in neutral at around 70 and let the transmission unload itself a little, and it seemed to subside somewhat.
I haven't had a chance to get my front end in the air again and give both wheels a pull (I diagnosed by bad tie rod because I could knock my left wheel back and forth almost a centimeter with the tires off the ground) but I really doubt the few weeks when my new tie rod was on there that it somehow got its ball joint rattled loose by my bad left front wheel, as what happened to the old one. If it did, there goes an afternoon, but the part has a warranty.
Still, I'm not expecting to find any wiggle, which leaves me wondering where I could get an intermittent vibration from. It never comes and goes on the same stretch of pavement, the car generally seems to have to be unsettled by a change in the pavement somehow. I know that the left-hand driveshaft on this car is really long compared to the right one, perhaps there's some kind of rubber balancer donut it needs to keep from jumproping around or something?
I'm planning on replacing my cap and rotor tomorrow before work and will jack up the car just to give both front wheels a tug, I'm just not sure what else to look for in the event that the steering is solid on both sides.

[Eye of the Tiger]

I could think of some things, but since it's intermittent... I have. No idea. 

[Pancor]

Pay $20 to have your front tires balanced.  Believe it or not, vibrations due to tire imbalance can be intermittent.  Also, try swappning your front wheels with the rear.   

[Secret Chimp]

These are brand new tires, they just got balanced and installed a week ago.

[Pancor]

These are brand new tires, they just got balanced and installed a week ago.

Doesn't matter!

[S204STi]

These are brand new tires, they just got balanced and installed a week ago.

My brand new tires also vibrate, so take Pancor's advice or know the reason why.

[omicron]

Pay $20 to have your front tires balanced. Believe it or not, vibrations due to tire imbalance can be intermittent. Also, try swappning your front wheels with the rear.

The Lexcen developed shocking vibrations under light braking from 100km/h, to the point whereby the steering wheel was almost shaken out of my hands, which was eventually diagnosed as tyre imbalance on newly-fitted tyres.

[etypeJohn]

Not only balance, but the new tires may be out of round or have lateral runout.  Where did you buy them?  Take them back and have them checked