The problem can be the lamps are bad, the only way to tell is to remove the lamps, look for poor soldering works where the wires are attached to the lamps, bad lamps will have very dark color at the end of the lamps, just like bad fluorescent lamp you see when it is worn out, anoher sign of bad lamp is pinkish glow when lamp si first turn on. The transistors and caps are good in the inverter section?