Quoted from jkashani:
Understood, but it is only one row of lamps, the others are fine. Any other suggestions?
Ok, you did not say that you installed the 555 lamp boards from pinball replacement parts so I assume you still have original factory metal plates with the #44 bulb sockets in your Nine Ball.
Those metal plates get to be a pain with lousy connections. Could some of your sockets be corroding at those metal plates and be the problem?
If you have not installed a set of these I highly recommend you consider them.