I'm troubleshooting an issue with my Xenon. This is the first time troubleshooting a solenoid problem.

The issue:
Immediately after powering on the game, the left slingshot randomly fires sporadically. Sometimes it fires "quick and soft" while other times it fires nonstop and blows the playfield fuse (if I'm unable to turn the machine off in time). When powering off, the sling returns to its normal resting position.

Notes:
* This issue is occurs before starting a game, while the machine is idle.
* The issue can continue to occur randomly as the game sits idle.
* After Xenon is powered on, the top saucer solenoid fires, followed by the side saucer (to return any potentially missing balls). It almost appears that the left sling begins firing, as described, while the side saucer fires during startup (might be unrelated).
* No other solenoids are firing unexpectedly, the issue is isolated to the left sling.

What I've tried:
* I've removed the left sling coil from the machine:
** Tested it, installed a new diode, all look well.
** Inspected the coil sleeve and mechs, it doesn't appear to be a mechanical issue.
** The switches appear to be working as expected, I've blocked them while testing. The game isn't running while this issue happens, I assume the switches are unrelated
* Using the schematic, I've located the associated components on the solenoid driver board. Keep in mind I've very new to this and have little to no electrical experience.
** The J5 wiring harness appears to be securely attached as expected.
** The R31 and R32 resistors are reading 120ohms and 310ohms (consistent with the other components on the board).
** The Q11 transistor is reading 500ohms and 800ohms (consistent with the other transistors).

Questions:
* Can I test the CR11 diode while it's on the board?
* Can I test the C21 capacitor while it's on the board? I don't have an ESR meter on hand.
* Can you test a diode while it's on a coil?

Any advice on what else I can try, or what might be causing this, would be greatly appreciated!

Screen Shot 2022-07-06 at 4.42.34 PM (resized).png

I just replaced the rubbers. I will try to reproduce the issue with the rubbers off and respond back.

Thanks for the responses.

The issue was occurring long before the rubber was replaced. I mentioned blocking the switches in my troubleshooting. Would a closed switch contact even trigger a solenoid to fire prior to starting a game? The other sling doesn't fire when the switches are closed, before starting a game.

Here is a video of the issue, no rubber, switches open: https://drive.google.com/file/d/1kpsKh106dGtFmNp44fhXpyshWR6jzKgL/view?usp=drivesdk

Edit: a note, I mentioned the sling returning to normal after powering the machine off. This was with rubber on. Without rubber, you can see it remains out.

Thanks @bigal56, these are helpful tips.
I'll have time to continue my investigation next week. I'll be sure to follow up with my findings.