7.4 87 2500 Sierra Classic SES

[Pagj17]

Howdy All,
Trying to figure out what's up with my car is like trying to read a Moron's Mind:P
I love her and all, but she's making life painful
Alright now, to the problem.

I often get a check engine Soon light at around 65-70, (although today that didn't happen). When It does activate, I can slow down some, and it deactivates after a little while, (usually with in 5-10 minutes). It also doesn't happen at lower speeds.
There are also times, when my friends are behind me, that they smell gas not being burned in the exhaust(and presumably it's noticeable enough to cause an issue) and of course, My mileage is down to around 8mpg.
Any help is beyond appreciated, thanks Folks
Also, I tried to check the codes, and found quite a bit of help, but all it does is flash "code 12" and then a few times it flashed three in a row, then 2.
Thanks