1998 Mitsubishi Galant 1998 Mitsubishi Galant Transmission

Engine Performance problem
1998 Mitsubishi Galant 4 cyl Automatic 139000 miles

My girlfriend owns a 1998 Mitsubishi Galant ES with 139000 miles on it. It recently begun acting up as soon as I changed the oil on her car manually. The engine shuts off sometimes when she puts her foot on the brake at stop signs and red lights. I noticed that when it doesnt shut off, the engine gets exceptionally weak when pressing the brake.

Also, the transmission shifts really hard when going from 1st to second gear and from 2nd to 3rd (car is an Automatic by the way). Any suggestions on what is wrong?