THe typical ohv tstat starts to die the truck does not warm up as the tstat opens early. truck works fine in the summer but has no heat in the summer. I replace mine every two years for this reason.
If someone has used a crappy tstat they tend to not maintain the desired temp and can bounce around. It will be more noticeable as it get colder outside.
So with that being sad tstat and a coolant flush/fill should take car of the problem. If it comes back you most likely have a small leak in a hose somewhere letting in air. Plus it will lose coolant.
99 x-cab 4x4 4.0l auto
mods, cranked t-bars, silencer delete, k&n drop in, Rebuilt trans ,Trans Go stage 3 shift kit, 3200stall Fuddle Racing Torque Converter,
VANIR 63mm TB, Roush split rear exit exhuast, E-Fan, JBA Headers
Auto up/down both windows, OHC, EATC (soon to be modded)
03 Crown Victoria Police Interceptor