What would cause the cruise control to stop working at various times?

What is your question?

I travel a lot so I need the cruise. I've been careful to make sure I'm not hitting anything to cause it to disengage. Most recently I was passing an 18 wheeler with cars behind me when suddenly the cruise quit working and the car suddenly started slowing down. It scared me because of all the cars I had behind me and I wasn't expecting it. It's at the dealership right now and they say they can't get it to do it so there's nothing they can do. I'm scared to drive the car because with all the driving I do, I really need cruise. It happens every time I'm on the road but kept thinking I was doing something to cause it. I just had right shoulder surgery so I can't even use my right arm at all. I drive with my left hand at 12:00 position. There's no way I could hit the "off" button driving that way. I hate this car now because as much as I drive, I need to rely on the cruise and can't. Help!!! Please!!!