Of course the problem is intermittent.It once in a day or two sommetimes it works for 5-6 days continuously.
Relays are driven through transistors putting diode across the coil.
I have used timer0, and timer1 interrupts for timing the relays.
I have used IC7805 in power supply and simple 10v tranformer.
I have not used serial port at all,no timer2 is used.
Even I tried with the same code on 89s8252 with watchdog enabled,problem is still persisting.