As far as I remember this was the problem:

Power-up sequence:
Apply power between VCC and GND pins
Set RST and XTAL1 to GND

Which as also what I did - applied power first and then set RST and XTAL1 to ground afterwards. That worked with old chips, but doesn't anymore.

I have now changed the code so both the above is done in one step.

I could release my code, but I don't think the world need yet another AT89Cx051 programmer ;)

Best regards,

Mikkel C. Simonsen