How do I make my air conditioner's condenser stay on?

That may very well be the case: the compressor is controlled by a thermostatic thermocouple wire looking device that hangs down into the return air stream in front of the evaporator. When cleaning the front filter (you DO keep that filter clean, right?) sometimes people mistakenly push that thermocouple into making contact with the cold evaporator. That fools the thermostat into thinking it is cool enough and shuts the compressor off. I know you said condenser. (the condenser is the outermost radiator looking device on the outside of the unit)

Also, if your unit is not very very clean inside, it cannot cool to its capacity...if the unit has not been cleaned yearly, take the thing out of the window, set it on the tailgate of a pickup truck, take a few screws out, peel the skin and clean behind both coils. Then your unit will work like new.....