Having RPM gage issues

I have a 2002 Oldsmobile Aurora that I just bought 2 weeks ago. The check engine light was on when I bought the car but I wasn't having any issues.. It was put on the computer & it had a misfire code that came up. Then last week I went to start it & it immediately died.. I turned the key off & it started back up with no problem.. But my RPM's immediately went to 8 while I was sitting there waiting for it to warm up.. I didn't realize at first so I started driving.. Then when I came to a stop my RPM's was going down to 4.. That's when I realized what had happened..So I turned my car off & restarted it & then my RPM's were between 1 & 2.. It does this all the time.. Plus there has been a few times that I was driving & my RPM's were normal.. Then I give it gas & my car kind of jerks & my RPM's immediately go to 8.. So I have to stop to turn my car off & restart it again. My mechanic says that I need a crank sensor & that will fix the car from the dying issue.. But he is not sure why the RPM's do this.. Any idea's???