For the 876A try a bypass of 0.1uF ceramic (C2) and a decoupler of 10uF electrolytic (C1) as close to as possible to the VDD and VSS pins as shown. Every design is different and it depends on trace/wire lengths, other components, and everything else that is going on. (These are the values I start with and use in almost all of my designs.)
It is hard to say what to try with the power supply without knowing more about your wall wart and your servo. You mentioned 7V but nothing more. You could try adding one or more 100 – 470 uF capacitors in parallel with the capacitor on the “regulator” side of the regulator and even possibly a 100 uF on the other side.
You should note that most wall warts supply the rated voltage (7 V in your case) at the rated amperage (you did not list). Further, they usually supply a higher than rated voltage if loaded less and a lower than rated voltage if loaded more. If your load is less, then you will not have a problem. However, ... most 7805 regulators have a Vdrop of 2.0V, meaning if your wall wart voltage drops below 7 V due to a larger than rated load, the regulator will drop out (i.e., “turn off”). This too could be adding to your troubles. Can you provide more information on your servo and/or on your supply? You also can try the above. Again, let us know.
Good Luck,
Paul Borgmeier
Salt Lake City, Utah
USA
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