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Display would start acting up after a few minutes of game play then symptoms would completely disappear after 10-15 minutes.

An obvious intermittent fault due to heat.

Symptoms: Grey vertical lines mostly around sprites but also affected other areas of the screen.
Troubleshooting: Cooled off suspect chips ( I used some ice wrapped up in a plastic bag to avoid any leaking )  and found cooling the transistor @ Q4 on the video board would reproduce the above problems.
Fix: Replaced the faulty 2sc1815 transistor at Q4 on the video board which cleaned up the video sync signal.

Nice fix!

Namco Classics 1 and 2 have the same video issues at times, but those are related to garbage capacitors that need to be replaced. :)

Still one more issue to fix on this board which I'll post the solution to once I find out the problem.

Game runs most of the time when switched on, other times it doesn't start at all. So when this happens address pins on the 6502 are all stuck high ( 0xFFFF). Everything else looks fine via logic probe.. RST is high, R/W is high, RDY/IRQ/NMI are all tied to 5v and CLK is pulsing although I don't have a scope to see it clearly. It's a Synertek 6502, I tried swapping the sound CPU ( A Synertek as well ) but same issue.

Strange problem for sure.

Watch the reset pin with a logic probe. It's probably not working properly. Could be as simple as a bad cap in the reset circuit. Replace C49 and C48 and see if that fixes it.

I'll give that a go. Thx.

But I suspect the LS04 at 9C to be flakey.

Pin 8 on the LS04 is completely dead ( floating ) whenever I observe that fault. So I believe there is no trigger to start the 555.

I will replace LS04 and hopefully that will fix it.

I cannot believe this. 9C was indeed the culprit.

How this game even worked half the time is a miracle.
It was installed from the factory incorrectly with pin 7 which is
the ground pin completely missing the plate through hole
and crushed under the bottom of the chip. Unacceptable
really. The pin has no solder on it at all so it wasn't
damaged during removal, I confirm this because I always
snip the ic legs before removal as the solder braid covers a wider
surface area over the pad making removal easier.

Nice one :)

It's odd to find issues like that, but it happens. I was given a brand new 14" SVGA monitor once that had been sent back for repairs and not properly fixed. The distributor replaced it and said to toss the bad one so I opened it up and found that the bridge rectifier on the AC input side was stuck in at an angle and the + pin was barely soldered in.  a quick desolder, reinstallation, and resolder job fixed that monitor and I used it for years.

When a CPU doesn't start up properly it will act locked up like that. The reset circuit is always the first place to check. You should've seen the look on my face when fixing a bootleg Pleiades boardset where the manufacturer silkscreened AND installed a Tantalum cap backwards in the circuit. The only way I found it was tracing through the circuit to see a positive voltage on the - pin of the cap. If it weren't for the current limiting of the resistor in that RC circuit, the cap would've popped or caught fire long ago. Reversed the cap and the reset circuit worked normally afterwards.

That would have taken ages to find and that's an inexcusable mistake even for a bootleger.  In my case I think there must have been some solder in that hole so the chip wasn't pressed correctly and the pin collapsed underneath the chip, I assume that the ground pin still made a good enough connection to the pad for the reset to work half of the time.