SES light on - fuse 8 blown?

SES light came on before I put my car up for the winter. It didn't effect the driveability or anything, figured I'd get it look at when I got back on the road this spring. During the winter I was checking fuses and noticed fuse 8 (engine controls) was blown. I replaced the fuse and started the car up and it idle rough for a few minutes and smoothed out and the SES light went out. Took it for a short drive (less than a mile) - light still out - thought I solved my problem. I got my car back out a couple days ago and after about 5 miles SES light came back on. Sorry for post being so long, just want to clearly expain my problem. My question is???

What does this fuse 8 (engine controls) actually control?
Has anyone had similar problems or know what could be wrong?

My car is 2003 Mustang GT - manual - Stock (except stock mufflers are removed)

Thanks for any insight that can be given!

Thanks Dan! Just want to know what could be wrong and if I could fix it myself, I have a bit of mechanical and electronical experience and would rather fix myself than pay for it...just need to know where to begin?

Thanks! I'll do that.