Can an NTC sensor drift over time?
When using an NTC you should make sure self-heating is a non-issue by:


making the measurement current small so power dissipation can be neglected. It might help to choose an NTC with a high resistance value.
measuring at intervals and only providing power to the NTC when measuring so that it can cool off when not measuring


You could use an alternative component:

A PTC does not have this thermal runaway issue as it counteracts it's own self-heating

You could use a more complex sensor based on a chip, example: DS18B20