It could be a bunch of things, but a few common items to check include bad connectors on the power supply board, cold solder joints on the connector header pins on the power supply or solenoid driver board, or a failing 5V 12000uF capacitor on the solenoid driver board.

Most of these pinball boards are just plug & play. What exactly does "installed professionally" mean in this context? Not trying to be a smart-ass; it's a serious question.

Hopefully they did more than simply remove and plug in two new boards. I am not trying to be critical as they may have done a ton of honest work, but swapping out boards takes minimal skill and effort.

Would they be willing to troubleshoot this for you under warranty perhaps?

The board in the top right corner with the two heatsinks is the solenoid driver board. Rectifier board, sometimes called power supply, is in the bottom right corner, above the transformer.

You didn't mention which board you were testing voltages on. Every board in the game has test points. Link below should get you pointed in the right direction. If you don't have a game manual, you can download one at the second link.

https://www.pinwiki.com/wiki/index.php/Bally/Stern#MPU_boot_issues
(note the reset section in the link)

https://ipdb.org/

Your solenoid board still has the original electrolytic capacitors on it.
The big one in the middle is the filter for the 5V rail and if it is dying will cause you problems.
There are several Youtube videos on how to replace this and it is mentioned in PinWiki how to do this.
Macro Pinball even sells a kit of the 3 capacitors for the board.
Your board is also missing the perspex cover over the HV section of the solenoid board, so don't got poking your fingers near the top left of the board with it turned on unless you like shocks.
The part number for the HV shield is Bally M-1837 not sure what screws you need to fit it though.
If you still have the original connectors on your wiring they could also be reaching end of life and need replacing which is why reseating them fixed it.
I just picked up one of these machines, but mine needs lots of TLC before I can play it.

Your solenoid driver board (top right) needs new capacitors and ground modifications. The power supply board (lower right by the transformer) needs to be serviced. I see some hacky repair and it probably needs new connector pins. Your sound board (small board lower right under the solenoid driver board) needs new capacitors too.

For reliability, all of these 40 year old components need to be replaced. Pins in the connectors should be replaced too. Some people just purchase new boards. That is great too. But the connector pins should still be replaced. I'm not too far away if you want board work done. Just PM me.

If the batteries on the old MPU puked, it likely corroded several of the pins in some connectors. Contact your professional who installed the Altek board and ask if any connector pins were replaced, especially J4 on the MPU.

When troubles start again, with power off, remove the connectors on the MPU and LDB (Lamp Driver Board) and see if the pins inside the connector have green on them. If so, then you’re seeing corrosion from the original MPU, and needs to be repaired by replacing the pins in those connectors.

As others have stated, the SDB (Solenoid Driver Board) probably needs an update. Yeah, you could buy a new board, but to me, there’s a certain thrill one gets with a successful repair.

Learn a few new skills! You’ll never regret it.

Why not have a go to replace the capacitors and do the modifcations, worst case you damage the board and need to replace it, but if you are already thinking about replacing it then no real loss.
I made a point of not buying a perfect machine as I almost like the job of fixing them up as much as playing them.

If you have some time to (probably over several nights) there is a series of videos on Youtube where an arcade repair shop gets a Bally Star Trek pinball machine that had been shot and left in a falling down house and restored it back to running condition and just about everything needed fixing on it. It's worth watching to have an idea of some of the issues you may come across but hopefully not as extreme.

1) Any Bally/Stern tech worth their salt would have changed out the 5V cap minimum. minimum.

2) The factory Bally SDB are pretty robust- Change out the 2 caps, a few ground mods, reflow the header pins, check the display volts to make sure they are not out of spec and you are good to go. $10 worth of parts if that.

3) Having to repin the connectors would not be a surprise, 50% of the Bally's I picked up needed the job done otherwise erratic operation is standard.

The connectors are only rated for so many insertions, time and environment takes it's toll as well.