Doom878

Since I'm in Pittsburgh, my sister drives my car in Miami. When she drives the car at like 40-60mph depending on the road it's normal but when she slows to a light or stop, the car tends to bog and turn off. The lights are on full blast still and there is NO dimming. It actually turned off about 7 times in 2 days. She says the car sounds weaker, less peppier sound. I had a similar problem before last year where my car would do the same. I took it to the dealer and after checking the throttle body, spark plugs, intake if clogged, charge of battery, alternator and who knows what else. They found the problem to be the ground wire from my system was loose. (My friend who did the install swore it wasn't). Anyone know why this is happening?