Engine it started shaking and it stall twice while I was getting in a parking space?

I have a 2003 Chevy Tahoe a month ago I was having problems with it, it started shaking and it stall twice while I was getting in a parking space the third time the car wouldn't start I had to jump start it.I took it to the Dealership they told me it was some cells inside the battery they replaced the battery and checked the alternator and told me that everything was fine I paid almost $300.00 Dollars. Now i'm having the same problem if I make a stop or get in a parking space it stalls, the RPM needle increases and decreases, I don't want to take it back to the dealership cause they charge to much money $80.00 fo a diagnostic and plus what it needs so I hope someone can give an answer. It cranks and starts but when I stop and get in a parking space the RPM needle goes up and down and most of the time it stalls when I get in a parking space.