1994 Nissan Sentra Possible Engine?

I bought this car off a friend who is a mechanic. The car had set for 8 months so when I drove it the thing kept cutting off on me at stop lights etc etc. I had the alternator and battery checked and it came back ok. I took it to another mechanic I know and he said it was the motor and thats why it kept cutting off on me. He said the motor should be getting 2500 something I dont remeber but was only getting 20. I told my friend this and he said it was not a motor issue. So I took it to him and he adjusted the idle control and it doesn't cut off anymore but when I get it up to speed it just sounds like something is missing like a spark plug is missing. And the battery light and the brake light continue to come on, what could be wrong with this car?