Is my tranny sliping?

for some reason my truck when running 43 to 55 mph can not figure out if it wants to stay in overdrive or not. now i can stomp it 0 to 70 set the cruise and never have a single issue, but this hoe is driving me nuts going in and out all the time at those speeds. it is a jasper tranny that has a bout 30k on it.

thanks for the advice and help in advance.
Kyle