My problem is still very much alive.
I had turned off the clock for over a month.
Now the clock "locks up" within 48 hours.
It has locked up twice at 7 hours , 14 minutes (7:14).
The mux routine works fine but the ISR just stops updating timing registers.
(The varibles that handle this are bres_hi ,bres_mid ,bres_lo.)
(uC is 16F628A and TMR0 used ot keep time)