My O/D light flashes when I am driving it at.

My O/D light flashes when I am driving it at lower speeds, like around town, where I have to do a lot of stop and go. But if you cut it off and cut it back on, the light goes off. If the light is on the transmission will change gears without jumping but its harder for it to change gears(You have to press the gas harder) but it if it is off it jumps when going from 1st to 2nd gear and will get hung up untill you let off gas. So through my knowledge and research I've decided it could be the control selnoid, so I am trying to figure where exactaly it is located on the transmission before I get started changing it. Thank you