We ran a 2 wire

We ran a 2 wire bus to an EEPROM at 400KHz from an Atmel micro running at 8MHz clock speed & it is fine.
We now run the same micro at 20MHz, with the same 2 wire timing of 400KHz & the EEPROM does not work, although a clock chip on the same 2 wire bus DOES work...
With the 20MHz micro we see a spike on the SDA line after the 8th clock pulse, although we so no spike when we run the same micro on the same PCB etc.. at 8MHz.

Any ideas?