Try fault finding:
Remove & replace both new RAM modules to make sure they are each seated correctly. You can also try swapping memory slots.
If that doesn't work replace the both new modules with the old memory & check to see that the PC works correctly in its old configuration.
If it does then replace the old memory with ONE of the new modules. Check how the PC functions.
If OK replace the new, working module with the other new module. Again, check the PC is functioning OK.
Try both modules again or in combination with the old modules.
Just a thought, but are both modules the same as each other OR the old memory ie 66/100/133 MHz or parity/non-parity? Might make a difference.