Transmission problem
2005 Suzuki Forenza 4 cyl Front Wheel Drive Automatic 50000 miles

Sometimes when I go to put my car into reverse or drive, it will make a "thump" noise, like something is dropping and the car jerks slightly, and then it will act like it's a manual in gear but will not shift gears. It will remain in whatever gear it's in and the rpm's get really high, it's slow to start moving and it wastes a lot of gas!
please help!