Z axis squeal

[sjk]

My I3 pro B has worked fine for at least a couple of years but I now have a problem with the initial Z axis lift. Sometimes the print starts successfully but usually there is an unpleasant screech and the Z motors only turn a little - and not always the same amount. Of course the print then doesn't work

I see that near the start of the Repetier Host generated gcode for any object the lines dealing with Z motion are:

12 M203 X500 Y500 Z12 E120 ; sets maximum feedrates, mm/sec
...
19 G 28 ; home all axes
20 G1 Z5 F5000 ; lift nozzle
...
27 G1 Z0.300 F7800.000

If I manually edit lines 20 and 27 to use lower speeds, it all works properly but this shouldn't be needed because of line 12.

Because lines 20 and 27 only run the motors for a very short time, I tried changing line 20 to G1 Z60 F5000 so that I could see how long the screech would last. It took about 5 seconds (but no motion) which corresponds to a speed of 12mm/sec - i.e. what you would expect!

I am confused - I haven't changed anything that I am aware of. Using a M115 command I get:
14:58:39.465 : FIRMWARE_NAME:V1.0.03 Sprinter/grbl mashup for gen6 FIRMWARE_URL:https://github.com/Ultimaker/Marlin PROTOCOL_VERSION:1.0 MACHINE_TYPE:I3proB EXTRUDER_COUNT:1 UUID:00000000-0000-0000-0000-000000000000

What can I do to sort this out?

[LoBoHEAT]

Hi!

Check Z-Axis driver, voltage 0.8-1.2V
And most important, check T8/M8 rods, possible they bent and impossible to turn correctly.

I have the same problem after 200 hour of print, 20 days around 10 hours a day, the M8 rods were bend, I change to new T8 and fine.

Best regards