Are midges usually a problem in late August, early September?
It depends on where you are going. Most of the midges will be in the western Highlands and on out. I believe they do start to taper off in September, but will still be around.
Midges thrive in damp humid conditions and particularly around forests. They don't like wind so open places with a breeze won't be too much of a problem. You won't be bothered in towns.