You said you measured voltage....what are the exact numbers you are getting including AC ripple voltage?. MPU IJ17 Pins 4,5,6 = +5.00 VDC @8.1 mV AC ripple, and measure U26/U27 ROM Pin 1 no less than 4.9 VDC
From the repair guides:
Game Just Won't Power On Consistently.
System 11 games can develop a problem with the POR (Power On Reset) circuit. If this circuit isn't working correctly, ofen you won't get the CPU to boot-up consistently, or at all. This can mean a "7", "8" or blank in the LED diagnostics box (system 11) or 7 or 8 LED flashes system 11a, b, c).
One thing that can cause the POR circuit to no operate is out of spec +5 volts. This can be caused by a bad +5 volt bridge or filter capacitor. The easiest thing to do is to isolate the power supply from the CPU board. To do this, remove the CPU board from the backbox. Then power the CPU board from an external power supply (as described below). If the problem still exists (inconsistent CPU booting), the problem is on the CPU board itself.
To "go around" the POR section of the board, you can do a simple test. After attempting to boot the CPU board, ground the /RESET line on the CPU chip (pin 40 of U15) momentarily with a test clip. If the board boots up consistently without a problem (correct flashes or "0" in the LED), then the POR section is definately at fault.
The POR section's job is to look at the +5 and +12 volts, and make sure they have stablized, as determined by zener diodes ZR1 (1N5996A, 6.8 volts .5 watt) and ZR2 (1N5990, 3.9 volts .5 watt) before starting the CPU. This is done by transistors Q34, Q36 and Q38, but before these three transistors can work, capacitor C30 and resistors R55 and R56 provide a delay to allow the voltage to stabilize. Only then the /RESET line can be released.
Any of these POR components could be bad, and not causing the CPU board to boot properely. If the board boots inconsistently, first look at capacitor C30 (a 22 mfd 10 volt electrolytic). If you don't have any way to test the capacitor, just clip another 22 mfd electrolytic cap across the leads of the existing C30 cap (positive to positive, negative to negative, a parallel connection). If the board now boots up consistently, you have found your problem! If not, check resistors R55 (4.7k 1/4 watt) and R56 (10 ohm 1/4 watt), and transistors Q34, Q36, Q38 (2N4401).