The first thing to try would be running it indoors with a standard atx power supply. That will narrow things down quite a bit.
I've completed my carputer for my RSX for a couple of weeks now and it's been running ok except for one annoyance.
Everytime I do a cold boot (after it has been off for a long time, especially overnight) the computer will restart itself at least two times after loading into WINXP before it can finally load WINXP correctly and run my front end. It boots fine for the rest of the day. This only happens when I leave the car overnight and try to boot in the morning.
I've tried switching out memory but had no success, still the same problems. Could it be the power supply?
40GB notebook harddrive
Road Runner front end
M1-ATX power supply
Before I installed everything in my car, I was running everything off of an atx power supply. I don't recall having any problems. I also fed the M1-ATX with a separate 12V supply and everything appeared to run fine. I'm not running anything power intensive so I would think the 90W M1-ATX should be sufficient.