Trouble making networking work with National Semiconductor DP83848J PHY

The MII control signals will run at a much lower speed than the data signals. So maybe there is something wrong with the data clock?

Might be worth forcing the PHY to work at 10M - which will slow down the MII data clock. If you can, set the PHY's ANAR (autonegotiate adverstise register) to only allow 100M FDX since that will complete auto negotiation to 10M, if you plug into an old 10M hub it will autodetect 10M and the device driver may get confused (it will probably read stale data from the ANRR and set the wrong speed!).