Electrical Problem 91 420SEL

HELP! HELP! HELP!

Was on way home from work tonight and decided to stop at the store. When I go out, put key in ignition and turned to ON position. Paused momentarily to turn down the radio and then reached back for ignition to start car. When I turned key to start position, nothing and dash lights went out. When I released the key, radio came back on. I got a passer-by to give me a jump start. Car started but cranked very slowly. When running, no alternator light and all gages were working correctly. Shifted to drive and went to take off -- engine sputtered, misfired, and then SRS light came on briefly along with the buzzer. I noticed that the radio (Becker) now said "CODE". Reset code and radio began working.
Decided to hightail it home and had no problems on the way home. Pulled into the garage and turned car off. Immediately tried to restart but same as above.

I have only owned this car for 1+ years and do not know how old the battery in it is. Do you think the battery is shorting out or has eeked out it's last amp?

Any assistance/solutions would be greatly appreciated.

Thanks in advance