Hi, Malc

@ First sight ... I could think the DS1820 do not update its scratchpad for any reason.
I read you use fluorescent tubes for lighting ... that rings my bell and I ask you

1) Which value for the pullup resistor ( usually 4.7k ... when sensor close to processor ! )
2) do you use ground shielded wire to connect your DSs ???
3) could you post me the DS identification and temp reading section of your program ? ( to see if any " health " control of the sensor is done )

may be we could think to some enforced DS chip " good running check " ...
some control like " heating power always rises, but temp not = default somewhere )

Alain