If your -17 MPU has the orangish brown sockets they are garbage and need to be replaced.

They are also a bitch to desolder. I wouldnt attempt this unless you can desolder very well. Need to pull the entire socket off at once, no pulling one pin at a time on this design.

The 555 almost never fails. Infact i do not think i have ever seen one bad on a bally board. I only ever replace them when they are corroded.

What i would suspect....

Low 5v
Bad 12v / 5v filter cap (measure 12v on rectifier and driver board should be same and slightly high)
failing ground input to the driver board for the 5v regulation (tie this to master ground like shown in clay's guide)
Battery damage causing high resistance somewhere on mpu
U9, U7, U8 sockets failing. Once get warm connection breaks.
Failing IC that overheats. I have seen a 6800 and a 6810 get very warm and hang the software.
Try swapping around ICs

Are they the orange / brown sockets? If so i can almost guarantee that is your problem.

Q5 getting toasted by the 82ohm 2watt resistor is another good one to check. Every bally board i touch gets that resistor replaced.

C16 and a resistor (1k i think) control the display interrupt timing. They get chewed up by corrosion and i have even seen a bad C16 cause the game to run slow due to it flooding the cpu with interrupts.

Could it have been .30v? If so that cap is on its way out and needs replaced.

Q5 fails all the time and i find them replaced at least one time on most bally boards. Probably from getting cooked by that resistor.

It is not going to be the roms. you likely have a soldering mistake at U10.

Now you need the test rom. With the test rom probe all the PIA outputs. Look for one stuck high or low. If you find one stuck high or low trace out that line and look for a short.

If None of the outputs are going high and low then one of the data or address lines is probably floating.

Use you DMM and check continuity from U10 to another IC.