Hillbilly Psycho

Please Help!! My father bought this van, and it had been sitting for quite some time, but it starts right up and purs like a kitten. The problem begins when going uphill, or when I kick it in passing gear. It will gradually lose power going up the hill and then will start lunging forward. I use the word "skipping" to describe what it's doing because it just keeps jumping over and over all the way to the top of the hill, but then it stops at the crest of the hill and runs fine until another hill or mountain, but as soon as it starts up the next hill, it does it again. When we got it, it had no transmission fluid in it at all. My dad drove it about 120 miles like that too! When I found that it had no tranny fluid, I filled it. We drove it around a little while and it was fine. Dad left in it again, and once again, it began to do this exact same thing as soon as he started up a hill. Anybody know anything about this? I changed the fuel filter, changed all the rubber hoses on the gas lines, put in fuel treatment, used Sea Foam, changed plugs and wires. What am I missing here? Thank you in advance for any help that you may give me. It's greatly appreciated.