95 e150 transmission

I have posted a thread on this before but so far I've had no responses. I would really appreciate some help on this. The symptoms on this transmission are sudden shifts from 4th -overdrive at hyway speeds. It almost seems like the transmission is in neutral, as you can press the gas pedal when it is doing this and the engine can be revved. then all by it self, it is in 4th or 3rd ratio??

Please help if you have any suggestions. I'm in Mexico and cant find a Haynes manual for this vehicle.

Thanks, Terry