Appears Lamp driver Q17 is for the Vari-target reset. Playfield mounted transistor.

http://pinwiki.com/wiki/index.php?title=Gottlieb_System_1#Remote_Mounted_Transistor

But did you check the contacts related to the varitarget?

Last part of Pascal's message:
"so in your case, it's related to the coil driven by lamp output #17 = varitarget reset coil: one of the varitarget switches remains closed.
>>>> It is a common problem with varitargets in general, they don't always go back to resting position properly and/or the several contact blades are misadjusted or get dirty. <<<<"

It could be due to dirty contacts where it registers in a static position during testing but doesn't always register during actual operation.

I would suspect his board last... it is possible that it could have taken a hit but check the inexpensive and problem prone stuff first.

Next follow the wiring and check the edge connectors (big System 1 weak point).