Diesel Bombers - Power cut out when pulling a hill

Ok, I have had this pickup for all of 2 months now. Just took a trip and had some problems with it.
Pickup was running fine running down the road. I had just left the kids house and had been running about 20 minutes. Doing about 70 mph and when I came to a hill, it cut out while pulling the hill. The RPM's dropped in a real choppy manner from about 2200 down to 1800. I backed out of it a bit and it pulled the hill just fine. If I pushed it on the hills after that it would cut out. With each chug there was a puff of black smoke at the exhaust.
I bought and replaced both fuel filters thinking maybe that was the problem. Pickup ran fine after that till I once again started pulling a hill. I was in construction so I didn't get a run before climbing the hill. If I pushed it at all it would cut out and do the same thing as stated above. If I got a decent run at the bottom of the hill it would hold 70 mph and could even gain a little if I asked but didn't push it.
On flat ground if I stomp on it there is no problem. It kicks down and takes off.
It is using? antifreeze. I say using because there are no drip marks under it. I have added 1/2 gallon on two different occasions. No antifreeze in the oil.
Pickup hasn't overheated. No check engine lights. The only warning light that comes on is one that shows a battery. I had O'Reilys check it out and one of the batteries was dead. It was just over a year old and Napa replaced it. ( I know batteries should be replaced together. That will happen as soon as I can afford 2 new ones.) Replacing the battery didn't stop the light from flickering on and off.

I had a 03 Yukon gas burner that did this same thing and it was the catalytic converter that had plugged off at 186,000 miles. Is it possible this is the same problem? Any help is appreciated.