You may want to check the option control register bit 3 and make sure the internal watchdog is disabled.
From the user manual page 16...

"The DS89C420 provides user selectable options that must be set before beginning software execution. The Option Control Register uses Flash bits rather than SFRs, and is individually erasable and programmable as a byte wide register. Bit 3 of this register is defined as the Watchdog POR default. Setting this bit to 1 disables the Watchdog reset function on power-up, and clearing this bit to 0 enables the Watchdog reset function automatically. Other bits of this register are undefined and will be at logic 1 when read. The value of this register can be read at address FCh in Parallel Programming mode or executing a Verify Option Control Register instruction in ROM Loader or In-Application Programming mode."