I checked the connections and took off the RAM (2 x 32 MB), the CD-ROM unit and the Ethernet transceiver, it still doesn't work.

I don't think it's a hardware problem since Mac OS 8.1 on another partitition works fine, and also 7.5.5 which was installed before. I erased the hard drive before I installed 7.6.1 and 8.1.

On another forum someone told me he had the very same problem with his 6100. He eventually found a 7.6.1 version which worked fine, but he has no idea how the problem fixed itself.