culturing trouble making competent cells - (Oct/24/2007 )

I'm trying to make JM109 competent cells. I have good colonies on M9 + B1 plates that grow beautifully overnight in 2.5 mL LB. However, when I inoculate them into the 250 mL LB + 20 mM MgSO4 they stop growing, utterly. I've tried starting from new colonies, using impeccably clean glassware and at 18, 20, 22, and 37°C. I'm baffled. I've never had this trouble before. Can anyone offer a possible reason for this, and, more important, a solution to this issue??

Do you have phage contamination in your lab? Could you try disposable flasks for the growth to test for this? Do they grow without the MgSO4? Could the MgSO4 be contaminated with phage? It is often used in running phage experiments.