Scanner to it and he said it kept showing that ECM was getting weak?

I have a 96 s-10 with a 2.2 L motor. I just bought it and got it for a very cheap price. It runs very, very rough. The man I bought it from said he had a friend with a handheld scanner hook his scanner to it and he said it kept showing that ECM was getting weak. I bought it thinking that a ECM shouldn't be hard to find for it. I did find out right away that a 1996 is the only year that will interchange with it. I've been told now that when I find one it will have to be flashed. If I find one from another 1996 s-10 with a 2.2L, will it have to be flashed? I would think if it is an exact duplicate truck that it wouldn't. There are several 96 GM cars that will interchange also. If I get one from a salvage that interchanges will it have to be flashed? Is the dealer the only place to get them flashed? Please help, this is all new to me