So here is what I've found:
Looking at the schematics, the tip42's with the troubled lights are Q52, Q54, Q56 and Q58. None of the other lights have issues. I tested those with a voltmeter and they do not appear shorted, or test any differently than any of the others - i wasn't expecting them to.
Looking at the schematics this afternoon before I got home, I was expecting it to be the 7408 (U52) or the PIA (U54). However, after getting some readings with a logic probe once I got home, either this doesn't work like I think it does, or i'm really confused or both.
With the machine on single lamp test on lamp #33, the reading off the PIA on Pins 14-17 (which feed the 7408) were all H/L, was expecting pin 14 to be High or at least different somehow as it's the only 1 of the 4 that should be 'on'.
On the 7408,
Gate 1 (feeding TIP42 - Q58) with pins 1,2 and 3 - the readings were 1-H/L, 2-High and 3-Low - this seemed ok.
Gate 2 (feeding TIP42 - Q56) with pins 4,5 and 6 - the readings were 4-H/L, 5-High and 6-Low - this doesn't seem right as this shouldn't be energized or at least different that above which should be.
Gate 3 (feeding TIP42 - Q54) with pins 12, 13 and 11 - the reading were 12-High, 13-H/L and 11-Low - Different than two above, but still shouldn't be energized.
Gate 4 (feeding (TIP42 - Q52 with pins 9, 10 and 8 - the reading were 9-High, 10-High and 8 - Low - Different still for them 3 above, still should not be energized.
Dont want to just randomly be replacing things, but really confused at what this is telling me.
Any help is much appreciated.