Stephane,
Go ahead and solder your chips, I think we can trust Philips, I am now soldering the PLCC devices directly on to the PCB, I seem to be getting consistent and reliable results.

I once had similar problems to what you are facing now with the Aduc812 from Analog Devices, when I contacted them they did not give me a proper answer nor did they solve it at that time, the flash used to get earased when the chip is power cycled, they might have solved it now.

Why dont you work with the P89C51RD2 devices than the P89C668 initially?

Regards,
John.