If you want to control temps you have to control the enviroment. If the shed is not insulated then your not controlling the environment. The exhaust system is not supposed to cool a room, its supposed to put 'fresh air' that has not been depleted of its co2 by your plants.
I am guessing your not air cooling your lights in a seperate system? The Air-Con is probably shutting off on Thermal Overload, it has a protection feaature built in, if the blower motor gets too hot, its shuts down so it doesn't 'burn out'.
There is not any difference between how you would insulate a house you want to live in and the shed you want your plants to live in, I bet your house is insultated. You don't think your plants deserve some too? Once that has been done, the only remaining difference is ANY HIGH POWERED LIGHTS, or more than normal for the size of the space, so you isolate that.
After thats done, if your portable air conditioner doesn't do the job, it doesn't have enough capacity for the size of the space with the ambient temperatures you have or you have worn it out by trying to have it do something its not designed to do. VV