Sounds like heat issues to me. You do know you cannot have the tops as close to the new light like you did with the CFL. When your light is on put your hand just above the tops of your plants and if its to hot to keep it there than to close for your plants. I have my 400w HPS about 12-14 inches off my tops. But I also have an enclosed hood with a fan hooked up to it.

S_a_H