What kind of containers are you using and how dense is the planting?
How do you water?
What is the temperature in the room?

I grow under lights and have not had problems with damping off (knock on wood - I'm afraid to even write that!). What you're doing sounds similar to what I do but these things may be different:

I use Fafard Germinating Mix. I use plastic liners and plug trays, mostly 100 cell plugs and 72 liners, a few 200 cell plugs. If I re-use liners or plug trays I scrub them with bleach before reusing them. I bottom water everything. I don't top water anything until it is outside and hardened off. I fertilize with VERY dilute fish emulsion.

Hope this helps,
Kirk

Kirk: Thanks for the reply. The temperature is 75 degrees in the grow area. All the flats are in 100's. At end of season we wash and bleach all flats before we store them away so, hopefully, we are not carrying anything over from last season. I bottom water when I fertilize, but have top watered sporatically with sprayer. I'll stop that! Julia

Hi Julia
IMHO lights should be about 1" above top of plants at all times. That's how I keep mine during winter sowing. Yellowing sounds like possible potassium deficiency. I really like Scott's Peat-Lite 20-10-20. I use a little of it with every dunking.
If you're getting alot of damping off then it sounds like you may be over-watering. Bottom watering is best without drowning the soil.
Trial and error is the name of the game
Cheers - Kim
Billabong Fresh Flower Farm

Does a steady temp of 75 F seem a little warm to others. It does to me. Altho you've been watching the watering, some combination of factors is creating a too moist situation.