Check the connection to the outlet. You may have loose connections, reversed polarity or damaged wire ends. Overtightening the wire at the screw could damage the ends, or cause them to detach from the screw. If you used the quick-connects at the back of the outlet instead of the screws, that could do it also. The quick connects should never be used for #12 wire. Too high a voltage drop.
All this will cause a higher voltage drop - which would cause the symptoms you mention.
All this will cause a higher voltage drop - which would cause the symptoms you mention.