2001 Daewoo Leganza Intermitent Speedometer-Stalling-Loss o

Hope you can help! Several months ago, my Daewoo's speedometer would cut out, drop to zero, bounce up and down or just stay at zero. After a few months of this, it got to stage 2--when slowing down to stop, the engine would stall but could be started right up again and I could drive on. After a few months or this we get to last week. I was driving for about 1.5 hours at highway speed. For the last 20 minutes of the drive the speedometer started to act up and bounce around and/or drop to zero and just act crazy. I was driving down an exit ramp from the highway and with the slowing of the engine, the car stalled. I was able to restart the car, but once I attempted to put the tranny in gear, the engine would stall. Also, the power door locks stopped working. The car sat for about an hour, and I noticed that the door locks were working again, so, I tried to start the car. It did start and when I put it in gear it stayed running. I dropped off the car at the dealership to see if they could determine what was wrong. Here I am 5 days later, they don't know what is wrong with it. They have test driven it for up to an hour on 3 ocassions and sometimes see the speedometer bouncing around and sometimes not. It has not stalled for them and the door locks are working fine. Any thoughts?