Hi Alex, Welcome to pinside!!

Intermittent problems can take a little more persistence to fix because we're never 100% sure when they're fixed.
With that said, Inspect and diagnose this circuit with Alligator clip jumper wires
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features

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Note that using jumpers is different than using a multimeter. The top link in @HowardR's post provides a how-to.

Until then

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

Not sure jumpers are going to be of much help to you in this case, since the problem is intermittent and infrequent. I would just examine, clean, and adjust the 3 relay switches (H,Q & AX) in the circuit HowardR posted. You'll probably find one of them barely closing or having a loose contact.

Besides the schematic? NO sw. generally have an extra spacer.

FYI, all switches on a relay must change state when the relay goes between its powered-on and powered-off states. With the game off, you can manipulate each relay by hand and visually inspect for all of their switches to change states. While you are in there you can look for switch contacts not fully opening or fully closing, dirty contacts, bent switch lug tabs possibly touching each other, switch wires with retreating insulation possibly shorting, stray wire strands not soldered or possibly shorting to other things, etc. When two contacts are adjacent or barely touching each other, that is not necessarily a good enough closure to pass current. You should see both blades move slightly, before one pulls away from the other to break contact. Also a closed switch may not be conducting if the contacts are dirty. It can be helpful to stick a small strip of white paper behind the switch blades in order to see things better.

Your schematic has a table listing the relays. The table columns are INDEX (where on the schematic the relay's coil is located), NO. (relay identifier), COIL NO. (part number for coil replacement), TYPE (relay type), CONTACTS (number of contacts), USE (function).

The 3 relays mentioned above have switches as follows :

H: 1A, 1B, 1C
Q: 3A, 2B, 1C
AX with AXR: 1A, 4B, 2C

A=normally open, B=normally closed, C=make-break.
So Q relay has 3 normally open, 2 normally closed and 1 make-break.

The H and Q are both "AG" type relays which are the most common and simple types. The AX is part of a 2-coil interlock relay type. These relay types require a second coil to fire to "reset" the relay. The AG relays "reset" simply by relaxing when power to them is removed. The reasoning for the interlock relays is that they can hold either state, even when power to the game is off.