Thank you all very much for your useful replies. I think indeed the crystal is probably not fundamental then and also like Kai said the PCB is incorrect for this purpose. Because of the cost of the other kinds of PCBs I will just use a crystal of 16 MHz. 8 MHz just doesn't seem to cut it here as my serial interrupt code is using more than 29 cycles. 12 MHz would also work for my purpose, but it doesn't seem to be possible to generate the correct baudrate for DMX using that with a 8052.

Thanks again, this really helped me, otherwise I would never have figured it out.