Before buying a new tranny measure the voltage at the tranny terminals and see if it drops when the flippers are used. It may be pretty quick and hard to see.
What is the size VA rating of your current tranny?
Also try measuring the voltage from the tranny terminals to the light, it should be zero or very low, if is 0.5v or more then you may have a bad joint.
If all else fails buy a new transformer.