B&D CMM1200 cordless electric mower starts but keeps shutting off.

Howdy, I have a Black & Decker MM850 electric mower that starts, then stops a short time later -- anywhere from few seconds to a couple of minutes later. It winds down slowly when it stops rather than cut out immediately. It does this 3 or 4 times, then after that it doesn't start. THis happened last week. Today I had a little time so I took apart the start lever assembly on the handle, didn't see anything unusual, so I cleaned the plug and reassembled. It did the same thing as before. I've had this mower several years, very reliable up to now. Thanks in advance for any help, Cliff I have the cordless electric mower- CMM1200 - and it started doing the same thing. It starts up fine, then after about 20 minutes, it begins to wind down and shuts off. It will start up again if I give it a rest for a minute or two, but I have a lot of land to mow and this drives me crazy. I just submitted this question to the Black and Decker website and will post the answer if I get one that's helpful, but I'm sure they'll just tell me to take it in to be fixed. Grrrrr! susan