Could be a wide range of things.....
Circuit seperation, common to have lights and plugs on the same circuit, but contributes to this problem.
Loose connections, anywhere, or everywhere all the way back to the transformer. Bad CB's too.(When your lights dimm from someelses house... that's a problem!)
Undersized wiring....
You mentioned all motor loads, and if these are all from different circuits, and tightening connections in your house doesn't help, like iwire mentioned try to get the PoCo to check theirs. These type of things can be really hard to track down sometimes.
