1998 Nissan Frontier AC issue

I have a 98 Nissan Frontier 4x4 king cab\extended cab that has an ac issue. When I am driving around it always starts off blowing cold air. After about 10 minutes is starts blowing hot. After another 5 minutes it will start blowing cold again. It does this off and on the entire time. I find that when I am stopped it blows hot much quicker. Do y'all know what that might be?

My brothers 2003 does same thing and his compressor is shot (he took it to shop and it had low pressure). I was looking on forum to see if anybody knows the steps to replace his compressor. His is a 4 cylinder.