Trac active

Ibought a ford winstar 2003 -i have all the manuals the problem is I will be driving at speeds up to even 40 and the trac active will come on and slam on the brakes by itself and it repeats it a couple times in about 15 seconds - it will happen 3 or 4 times in a week and then wont do it for a long time - I have taken it to a mechanic and it doesnt happen when they drive it what could it be? Doesnt seem safe I dont have alot of options im a female with small amount of money