1998 Ford Taurus Car shakes and shuts off from time to time

My 98 Taurus has 200,000 miles. The last couple of weeks its been shutting off from time to time. ON the highway it runs smoothly, but on the street it sometimes idles rough and when I make a stop it wants to shut off, sometimes it does and sometimes I manage to get to work without having to turn it back on. Also my Overdrive light starts to blink from time to time, but not as rare as the idling problem. Any suggestions? The car has a lot of miles but it has been very reliable and I don't mind spending a few dollars on it to fix it up and have it running for a few more months till I have to get a new car. There is no Check Engine Light and I have been suggested by a mechanic that my transmission is bad, but I am highly doubtful. Any suggestions?