Arduino mega2560 with ramps1.4 - when switch from 5V to 3V3 thermistors not work

Please help.

When I'm using Arduino Mega2560 with Ramps on logical voltage 5V my both thermistors for printer 3D working (for extruder and heatbed).

but when I'm switch 5V to 3V3 - everything working well except my two thermistors. It show on LCD display only 'def' and 'def'.

So for sure thermistors (100k) are OK but only on voltage 3.3V not working.
What is the reason ? What should I do to solve this problem ?

Maybe someone will ask me why I choose 3V3 and not 5V ?
I have general supply of Arduino 13,5V and 300W to speed up heatbed.

but sometimes my printer 3D I switch on and no result - LCD empty, I haver to switch few times my printer and than it is ok. I have low temperature in my garage sometimes 5 degrees - maybe this is the reason ?
and sometimes on LCD display there are some mistakes in displaying.

When I switch to 3V3 everything working OK except two thermistors - which are 'def' and 'def'.

Thanks a lot for any suggestions
Best Regards