If you suspect that the 7805 is the problem, then why not just take it out and run the 5V rail only from USB. As long as you have both USB and 12V connected the Sanguinololu should still work fine. You can then connect the bare 7805 to the 12V and GND (leaving 5V output open), if it still gets hot, you'll know that the 7805 is the problem.
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