Which is more likely.......

When I turn the headlights on the pass side fires all the time, but the drivers side doesn't fire but half the time. Turn the switch off and back on and both fire and stay that way... no issues further.

Is the bulb more likely to be bad and not catch that first spark, or the ballast not likely firing the bulb on the first "on"?

 

Sounds like a ballast issue to me. But if you want to be sure, just switch the bulbs around and see if the trend follows.

 

I had this happen on my friends Avalon. Turned out the fuse in the relay was blown. Went to Radio Shack and picked up another and it was fixed

 

yeah the bulbs are REAL tight in the is300. I'd rather ask first LOL. In consideration I am realizing the passenger side did this before I opened the a to paint them black. I might have answered my own question. I think the bulbs got switched previously. Dang now I gotta swap them to confirm

 

Are you sure you didn't swap the ballasts too?

I've never had a xenon bulb die on me but sounds like you have an electrical problem more so than a bulb problem. I'd guess that it's the ballast or the relay. And if I had to pick one, I'd say the relay.

 

the ballast usually gives two sparks. for instance, without the bulbs, the ballast will spark the wires 2 times to insure the bulbs ignite, however if the connection from the ballast to the bulb is not secure, it will not ignite the bulb and a flickering light will be appearant.

in most cases it can be a wire harness issuse, fuse issue ex( 15amp) instead of (20amp), loose wire, etc.