all 3 machines are using old kernels, since all 3 are atlhon with 1 gb of ram bfs is a good option but the last one can use the a64 kernel so all should be using 188.8.131.52-pclos3.bfs and/or .a64 for the last one
all 3 are using nvidia video cards but not sure if the last one can use the current driver, other two maybe older version, there was a post talking about that on this forum
some cards will work weird if you don't set the right driver for them, nv could be used for all 3 but if you can use nvidia proprietary is better
the first two machines are old, 9 years more or less, have you done a memtest on those ram modules?
about maintenance, have you cleaned the machines? verify that the ram modules are properly inserted on the slots, same for video cards and no dust on the slots
the fans, temperature on the units, if there is much dust in them that can be a problem
psu, is the fan spinning properly?
can you open the bios and verify voltages on the psu?
all 3 machines are old, hard disk failures can't be ruled out easily here, i never did a hard disk test on linux, fsck is the command afik but i don't know the complete procedure
on old mainboards is common to find damaged capacitors like this
if that is the case, this is a reason for random lockups, you can change those if you are good with the soldiering iron but i wouldn't bother doing it, when one or two fail like this, the rest will fail soon so i usually recommend to replace the mainboard completely
video cards could have those too, verify them too
old psus can have capacitors damaged too, if you can open the psu and verify that, it will help but if you never opened a psu or those psus specifically i just would change them, probably both first are damaged or about to fail
the third mainboard could have the same problems so you could check mainboard and psu, since you are using the onboard video card nothing to verify there
on old mainboard with athlon xp processors the temperature can be a problem, the thermal paste probably doesn't exist anymore on them,so if you see that the temperature of those two in the bios is higher than 50 Celsius that could be a problem
if you have replaced the thermal paste before, it is easy to do but the problem is remove the heatsink without breaking those unshielded athlon xp(i broke mine 10 years ago
) because most of them are really hard to install and remove
i wouldn't bother verifying the thermal paste on the third machine, it is probably in perfect shape so just verify in bios the temperature of it, should be between 35 and 43 Celsius