Stewart:
Just as a point of investigation , why don't you measure the current during the time RESET is asserted. (i.e. hold reset on with a jumper and then measure current on PSEN). If the current exceeds the data sheet value of 650 uA then there is something wrong with your part.

All bets are off after 24 clock cycles becasue Atmel specifically states to remove the low on PSEN after the 24 clock cycle period.
Mike