I used to have the same problem, sometimes the temp would drop close to 60 on colder days. Replaced the thermostat this weekend and it is dead on 90* (have heard Behr is a bit on the hot side) all the time with the exception of sitting in traffic this afternoon where it began to rise in the 90-95*F heat.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth