Who does know a lot about automatic trans.?(Trouble with a non Mopar truck)

[COKE]

Hello,
I,m having troubles with the tranny of my 88´Blazer K5 6.2 Diesel,and due it is not a Mopar truck i did belive  this was the correct board for the question.

I have this truck for 5 years now and it was running strong until now.
Now the truck does run 5 miles or so and then it starts to loose power until it has not force for run and stalls.
Then the engine  starts again,but stalls when it starts to walk.
It seems like if it would  not downsift.I belive the "D" or the "OD" stays once it has been shifted and it does not downshift when the speed goes down or you start to climb.Then it has not force for move the truck in that ratio and stalls.
Even shifting with the column lever it does not downshift.

I put a new TV cable a couple of years ago and it has been working fine.
I have checked it and it seems to be fine,it moves freely and returns ok.The tranny fluid and the filter has a couple of years too and very low mileage(this truck runs 3k per year or so)

I don,t know very much about automatic transmissions and i have not idea what coould be the trouble.
The tranny is a TurboHidramatic 700 r4.

Any comment will be apreciated.
Thanks!

[Plumcrazy]

Sounds like you may have an engine problem not a transmission problem.

[COKE]

I,m also was beliving it was a trouble from the injection pump,but i really don,t know what to think.
The truck starts fine and does not throw  bad smoke.
I have checked the fue filter  and the lift pump and all is ok.The fuel is also clean without water.
When an injection pump is bad the engine does not start or throws smoke...This just starst to loose power until it does not run and stalls.

Does know someone what kind of checking could i do to know if the trouble comes from the tranny or from the engine?

Thanks.