P89C51: VOH @ VCC = 4.5V and IOH = -30 microAmp.

The data sheet for DM74LS573 says for High Level Input current :
IOH = 20 microAmp ( and 9 such were being driven by the 89c52!)

The question is, without the driver how did the set up work at all ?

1) the '51 does nor drive IOH, It drives Iih.
2) are you sure that there are no pullup resistors on the port
3) The fact that only 30uA is a vailable at Vcc-0.7 does not mean that 300uA is not avialable at Vcc-1.0

problem of random display jitter, I looked at the circuit and found there was no driver like 74244 for the common segment bus.

You can not imagine how many problems exist becuase someone decided to 'save' a chip. However, if the LEDs and the driverchips are on the same board, the cause is more likely due to insufficient decoupling than to a 'missing' driver. Put a scope on the pins and have a look. if the pulses do not exceed Vih for the drivers by at least 1/2V, you must either add pullup resistors ir a buffer chip, if they do your problem is elsewhere.

Erik