The transmission is going into "limp mode" . . . that's when it stays in second gear and won't shift up or down. If you can "catch it" when it's in limp mode, does it throw the check engine light? If so read the trouble code at that time and advise.
FIRST you need to check the transmission fluid level. If it's a bit low it will cause this problem. It is common for the connector on the transmission to develop a leak (the OEM 0-rings deteriorate). If you have an obvious leak of transmission fluid, it's probably coming from the connector so you need to replace that part. If the fluid level is correct and there are no obvious leaks . . . then . . . read on . . .
I had this issue recently but mine would not come out of limp mode no matter what I tried. I was getting a CEL and a trouble code of P0715. After installing a new conductor plate the problem was still there. It turned out to be a poor connection at the transmission control module (TCM or TCU) which is located passenger side upper floorboard. Grab the carpeting at the top and peel it down. Remove the white styrofoam piece and you'll see a metal plate. Remove the three 10mm nuts and lay the plate down on top of the carpet. The TCM / TCU is the black box you'll see mounted at the bottom of the plate to the left of the stereo amplifier. Try disconnecting that wiring harness from the TCM / TCU, clean up the connection, apply some dielectric silicone compound available at NAPA (to prevent future corrosion at the connection), hook the connector back up and see what happens.
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If that doesn't turn out to be the source of the problem then as ala_xfire says, you might need to have a new conductor plate installed in the transmission (but normally if it's a problem with the conductor plate, it wouldn't correct itself by turning off and restarting the car AND it would throw the CEL and a trouble code).
Here is where I bought mine . . .
http://www.ebay.com/itm/251359627584?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Good luck . . . Please come back to Let us know the results . . .