I thought I had replied to this but don't see it. I'll try again.

Was there a great difference in day length between your basement and your garden? I read somewhere that it is not only heat that makes lettuce go bitter and bolt but longer day length.

Well, I have the lights downstairs from 7 am to 10 pm, so I don't think that should be a issue.

So they went from 15 hrs per day down to roughly 13? I would be interested to know if that would make any difference. Any experts out there care to chime in?

When you find out the cause please post it here. I have had that happen on occasion and it was usually with plants I had started inside too. At the time I didn't think anything about it, just chalked it up to bum luck so I don't know what the lighting specifics were in my case but I don't remember the temps being very much higher than other years where the lettuce turned out fine.

I was reading that ANY kind of stress will cause lettuce to become bitter and bolt. Going from 13 hrs to 15 hours might make the lettuce think it's summer, so it bolts.

Variety might make a difference, too.

Lettuce can take quite a bit of chill. I would sow some in rich soil in winter and see what happens. Planting in front of a south-facing wall might make all the difference in the world, too.

Sue