Reset check engine light

Reset by disconnecting black battery cable. Battery is one week old. Drove about nine miles and when I turned the car off the electrical just shut down immediately, didn't even let the steering wheel go up all the way. Started up again about ten minutes later and had to gun the engine to not stall. Left her running during the next couple stops covering about five miles. Check engine and variable speed control lights have now come back on. Next stop I tuned it off. Same issue. Started okay, thirty minutes later and drove fine. Drove twelve miles and again electrical just turned off immediately in garage. Alternator was replaced at about 180,000 and I'd driven about five miles when these two lights popped on. Called my mechanic immediately and he said don't worry about it. Now I find out I've been driving without valuable safety systems for two and a half years! Current mechanic says it's an emissions issue. Running Cateclean through it now to possibly solve the issue inexpensively. Puzzled about the electrical at this point and called to get her in. Thoughts guys?