1999 Audi A4 AC not cooling

My AC cools sometimes and does not sometimes. When it is cooling - it is very cold, but when I stop in traffic or sometimes while on the interstate it will automatically switch to "Econo" and the air is warm. When the air stops cooling, sometimes if you stop and turn the engine off and leave it for just a minute and restart, the air comes on and is ice cold. There is no set time it is not cold or warm. It may go all day and stay cold, or it may be cold, then go to "econo", then I switch off econo, and it may get cold again. I am going crazy with this thing and not sure if it is electrical or ac, so not sure who to talk to about it.