MBPro doing graphics test on screen at startup?

I've had a weird problem crop up a couple times of late. First time it happened on battery, I thought maybe the battery was drained or something. I booted and the screen started going thru what looked like a test pattern: red, blue, green, grey, white, then some vertical bars, then jagged vertical bars, then back to red, etc. Not what looked like a defect, as the pattern was too regular. I then plugged in and booted and all was fine. The battery was charged and next time I tried to boot from battery it did the same weird test pattern type of thing. I then plugged it in again, but this time, even when on AC power, I got the test pattern again for 2-3 boots. I was starting to get really worried, so started looking online for someone with similar probs. But nothing. While surfing from another Mac, with the MBPro in my lap, I thought I'd give it a try and boot from battery. And it booted fine! Since then, I've left it plugged in and haven't shut it down.

I noticed that my firmware was not up to date, so I ran that and it went okay. On reboot the display was normal. I have left it on since then. Wakes from sleep no problem.

Any ideas? I have been afraid to shut down before I find some possible remedy that solves it permanently.