Blazer trouble grrrr

Ok here's whats going on. My 1991 Blazer has been constantly shutting off while being driven the last copule months. I took it to a garage and the guy said the computer gave him many codes which he thinks was caused by water in the gas. He put two things of that dry gas stuff and it and one for me to put in again when i fill up again. A few days after I filled it up it still started to happen even after the 4th use. What do you guys think the problem is? Usually after it shuts off, u go to restart the engine and the check engine light comes on and results in little power but if you shut it off for a couple minutes its fine,most of the time. I have also noticed a lack of power especially on a upgrade. Any help would be thankful :-)

P.S. The last time a tune up was done was in march of this year along with some 3 step fuel line cleaner.

Hmmm... Sounds more like clogged injectors or a clogged cat...I would at this point take it to a different mechanic. When was the last time you did a tune up? Check your ignition module. (When they heat up they start flaking out)

A fuel pump overheating will cause this also.