1994 Nissan Pathfinder Problems with accelerating and hitti

I can't tell if it is the Engine or Transmission. But I just bought this pathfinder a few months ago from an older couple who kept it in mint condition. I have had no problems until now. Whenever I press on the gas it hesitates to accelerate as soon as it hits like 15 mph. So I have to let off the gas and put my foot back on it. That seems to work a little when I do that, almost as if was a manual transmission and it can't find the gear unless I let on and off the gas. Also when I get up to 30-35 mph it does the same thing were it doesn't want to accelerate anymore. So I let of the gas for a second and step on it again. It seems to be getting worse and I need to find a solution soon.
I had a problem like this with my old Toyota Corolla and it turned out to be spark plugs. But the couple I bought it from said they just had a tune-up. I checked all the fluids (Oil and Transmission) and they seem to be fine. I can not afford to take it anywhere to have it looked at because I am struggling financially with two kids. I feel like I am running out of time. My brother said he can fix it for me if I can find the source of the problem. But he doesn't know what it is. He doesn't really know a whole lot about cars. If anyone may have any suggestions I would most appreciate it.
Thank you.