For the clock, did you try _XT_OSC in the config? You could also try putting a resistor (say, 1k) between OSC2 and the crystal. Also, are you using a parallel cut or series cut crystal? Most PICs are designed for parallel cut.
For the thermometer, how are you measuring the temperature? Are you doing an A/D conversion via timing the charging of a capacitor? This could be affected by the crystal speed being "off" as well, or with a higher Vdd, the cap would charge faster, resulting in your readings being off.