Yeah, could be as simple as old brake fluid, which has taken on more moisture and is freezing. If you can get it somewhere warm enough to bleed, that my be enough to make it act normally again when it's cold out. My '96 runner did this, and new brake fluid fixed it... heck I rode in that truck this past weekend (now my brother's).
99 uzj100, gx470
05 525EXC, 01 XT225, 07 TTR90, 07 CRF50