Whats wrong with my 2006 dodge Dakota?

We have had our truck in the shop about 6 times since Oct 2016. Our check
engine light comes on one day & goes off the next. The dodge shop finds
something wrong every time, it has cost us thousands of dollars. Light comes
on we make an appointment by the time we bring it in the light is off. They say
they can't work on it if the light is off. The light came on the transmission
started to slip took the truck to another shop hoping a new transmission would
be the answer. They told us it was the computer, when they went to put the
computer in they said we already had a new computer...Take it back to Dodge.
Took the truck with a print out of the codes from the other shop, the truck has
been at dodge for over a week & they can do anything because the check
engine light is not on!!!! Very upset & don't know what to do.