Autoleveling will randomly fail

I have never really been able to get my bed level in a satisfactory way.

At some point, I started using autoleveling with a servo and mechanical switch. This has worked a lot better than trying to level by hand, but every 1 in 5 prints will still have an issue. The print head will either start too close or too far away from the bed. Either the print comes out perfectly, or it's a total failure on layer 1.

I tried turning on interpolation, and I upped the grid to 5x5.

I've tried the probe repeatability test, seeing no issues there, both stow and without stow. I currently probe without stowing. I also will only do G29 after the bed is heated up.

What could be the issue here? Either the print is perfect, or 1/5 chance it's a total failure, and I've seen both failure modes, ie, starting too close AND starting too far away.

You have done the hardest part in order to solve a problem you must ask a question.

Still need more information like this.

I have used Marlin 1.1.5 through Marlin 2.0.3 with Bi-Linear bed leveling, I save the data after I have leveled the bed which allows me to restore bed leveling data.
Restoring the Bed Leveling data was done with M420 S1 in the earlier Marlin versions then with " #define RESTORE_LEVELING_AFTER_G28 " in the newer version.
Leveling the bed with a 6 X 6 grid about one every month then always making sure I save the bed Leveling Grid using M500.

When I find that my Print is starting a little high or low I quickly use " #define BABYSTEPPING " to nudge the nozzle into place as to not have a failed print.
As all models always have a skirt on them this allows me plenty of time to adjust the nozzle into the perfect height using baby-steps.
Then will readjust the Autoleveling for the next set of prints, this is not an occurrence that happen often an have learned to adjust the bed level / Autoleveling quickly.
I have also 3D Printed a proximity sensor mount that comes with a adjustable thumb screw to allow me a super fast adjustment of the nozzles height.

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Computer Programmer / Electronics Technician

I'm using Marlin, the version seems to be called "1.1.x".

Leveling type is bilinear, 5x5 grid.

Before every print, I do:
G28
G29
M500 (to save the autoleveling data)
M420 S1 (to enable using autoleveling)

That is exactly what I did until updating the the Newer Marlin Versions that contained.

#define RESTORE_LEVELING_AFTER_G28

Now I just Print, and every once in a while do A quick Bed Level, Test and thumb screw adjustment

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Computer Programmer / Electronics Technician

I think #define RESTORE_LEVELING_AFTER_G28 is the same as doing M420 S1.

So perhaps this is an issue with the G28 that I issue before the print. This deploys the probe, and I'm thinking perhaps the servo could be slightly off, leading to both the "too high" and "too low" issues occurring randomly.

I'm guessing the important thing is to minimize the length of the arm, to avoid a multiplicative effect on any angular error.

Right now, the arm I'm using is fairly long, it was a random arm I found on thingiverse. I think this issue might be solved by using a shorter arm, and a more accurate servo.

I've been using the HXT900.

I'm thinking maybe I should try the TGY-50090M?

Am I on the right track or am I way off?