2000 Chrysler Cirrus

When I put the car in reverse it won't move it starts to sputter, shake as if it wants to stall, you have to step on the gas and still and shakes, and now today it did this again, and now in forward it started to shake
I have brought it in a million times and they have checked the transmission about 6 times ran it through a battery of tests it came back good, but they are puzzled can't figure out what the cause is
Help please I have being going through this for about 4 months now and its getting worse I am so scared to get stranded. The mechanics can't figure this one out and naturally it doesn't happen when I bring it in.
I am getting so upset and stressed out what can this be any advice?