BIOS decreasing RAM and causing a restart

[KashmirPan]

Hey there, so lately there is this problem that made my life so frustrating, my PC restarts during load (usually multi tab browsing) when ram usage is above 50%
i tested the CPU , GPU and PSU everything was fine

when i try memtest , the pc always restarts around test 4 (no errors tho)
the problem seems to disappear after physically reinstalling the ram and then entering bios to choose (save and exit)
but the problem always comes back after 4 hours or so

today when it happened i didn't touch the ram just went to bios and hit the (save and exit) option
tested with memtest and opened multible tabs everything was fine until 6 hours later when it happened again
and i had to enter bios once more

do i need another motherboard or there is a way to solve it?
the RAM slots and RAMs themselves are working fine as i mentioned after going into bios and pressing save

Things i tried :
using only 1 ram in 1 slot at a time (tried both rams ane both slots problem happens during above 50% ram usage even if i use 1 ram)
taking out the battery for 10 mins and placing it back again
cleaning ram slots and ram from dust

pc specs:
Processor : core i5 3470
GPU : AMD Radeon 3450 HD
MB : lenovo think center m72e
storage : sata ssd crucial 240 gb
Ram : Samsung ddr3 4gb
Kingston ddr3 4gb
PSU : Jumbo (local PSU brand) 600 W

Thank you!