2005 Mercury Sable TRANSMISSION HELP!

MY 2005 MERCURY SABLE JERKS WHEN I ACCELERATE AND ALSO WHAN I BREAK. AFTER THE CAR LEFT THE SHOP FOR BRAKE REPAIRS I REALIZED AT THE FIRST STOP SIGN THE CAR WAS JERKING WHEN I TOOK OFF. MY REPAIRS ONLY INCLUDED BRAKES, OIL AND FILTER CHEANGE. THE REPAIR GUY SAID TO PUT GOOD GAS IN AND RIDE IT OVER THE WEEKEND B/C MY FUEL INJECTORS WERE CLEANED AND THAT COULD BE THE PROBLEM. AFTER THE WEEKEND THE CAR WAS JUMPING WORSE, INSTEAD OF AT ACCELERATION IT BEGAN DOING IT DURING THE FIRST 3 MINUTES OF TAKING OFF. THE RPM GOES UP TO 2 0R 3 AND STAYS THERE FOR ABOUT A MINUTE AND THEN THE CARE SHIFTS INTO THE CORRECT GEAR AS I AM RIDING. PLEASE HELP!