EDIT: I re-read the part of my post you quoted, and now I think I understand better. Yeah, it certainly does seem to me as though there is a second problem, perhaps the thermistor is simply amplifying the problem because it is doing its job. I'll try and re-install the thermistor, and heat it up with a heat gun and watch for the game to go to the "only once in a while" resets, which would suggest something else.

Yeah, I robbed a spring out of another dead table for now, but I also have a new part on order. Makes me very nervous to eventually find all the other hacks on this table...

This is awesome! Thank you so much, I will reproduce this test and see what my thermistor's are doing.

I didn't know anything about this. Looks like a decent idea. I am kind of reluctant to slap a band-aid over a problem I haven't identified yet, but I am also kind of eager to get this machine playing right. I'm going to order one of these just to have around as a diagnostic at least, although I am not convinced that it is too much better than a DMM for diagnostics. Might be informative on the whole though. Thanks for the link!

brilliant!

So both of the thermistor's test fine out of the circuit, they start with around 3 ohms resistance, and decrease to less than one when I heat them up externally.

However, they still cause resets when either of them is installed. So, my brain is thinking that the reset issue is in fact involved with the function the thermistor is supposed to provide.

Is there another power buffering system in the 5v circuit besides the thermistor? Something else that could be faulty and behave like a thermistor issue? A cap somewhere?

This is the connector I presume you are asking about? How could it be wired lower than 120V?
IMG_20160205_120233_(resized).jpg

J101 looks great. (Actually, all the connections on the power driver board look good except for 1 of the GI connectors that was pretty hacked when I received it (all fixed now))
IMG_20160205_115833_(resized).jpg

I have reseated every connector in the backbox (multiple times actually). I also did a test (recommended by Zaza) to jumper past the Z-connector and 114 and 210. None of those seemed to help.

Wall voltage measured at the utility plug in the game is steady 118V.

I did go looking around at the transformer connector, and someone had installed a piggy back to provide line voltage for a bill acceptor (there is no bill acceptor installed in the game), and that piggy back seems to use smaller gauge wire for the jumpers on the transformer connector. Could reduced wire gauges there lead to this problem?

I just removed the bill acceptor connection and played a nice game with lots of double flipper action, but one game wouldn't always trip it. Also, I still have the thermistors out of the circuit (jumper installed).

IMG_20160205_123855_(resized).jpg

Hmm, haven't looked too much at connectors away from the board, but that makes a lot of sense. I will do that when I get back to the machine tonight.

I only have IDC Connectors at teh house, I'll see about picking up the tri pins for J101. It does still fell pretty snug to me. The only plug that isn't super snug is the 2 pin ground plug just above 101 (103 I think?) It is not the most secure plug. Worth replacing I'm sure.

There aren't any burnt connectors in the chain from the transformer to the board and I went through and reseated everything I could find several times.

I did start thinking about connectors though.

Could J121 or J115 have ANYTHING to do with th reset circuit? I think J121 is just GI lighting, but it had been hacked pretty awful when I got the board. The headers and the connector were both scorched and part of a trace on the back was starting to lift to due burn damage. Someone had clipped the wires off the connector and soldered them directly to the headers, and then just bent the header pins up so the rest of the connector could be plugged in. Pretty terrible. (But I don't think this has anything to do with resets...)

I don't know the story on J115, but I can see that someone has replaced it. The connector at least is not stock. No idea why it was changed or what that connector leads to (Just haven't looked down the line yet), but that repair does look sound without anything glaringly ugly.

I keep on thinking I'll find this mystery answer, but not sure. I'm thinking maybe I'll take the board over and slap it into a friends TZ or IJ and try to rule out all board issues.