During the middle of the day, the draw on the city grid can cause issues to. Peoples A/C's running etc can cause fluctuations in the total available power available to you. Could be part of the problem too. Hope you get it figured out..
Just as an example, my lights popped the breaker a week or so ago. Thought my A/C took a dump as a result. Turns out that the draw from all my equipment caused a spike at my main, causing the 50 amp breaker on the A/C condenser to pop. Reset it and all is right with the world again.... Except the electric bill of course...