At first, could not pull a boat. On an incline it acted like someone turned off gas, it would spit and sputter

For 6years moter runs like a brand new engine untill I try to pull boat. Over the years we have rebuilt tranny, replaced cats, o2 sensors, timing chain and gears, new distributer, cap, wires, plugs, new gas tank, fuel pump, fuel filter.I finally ran out of ideas and bought new truck!I kept the old truck to haul firewood as it ran like a brand new engine if not pulling boat. Someone suggested replacing fuel pressure regulator, the one thing we didnt replace.I had fuel press reg replaced and the guy took it for test drive, said it ran great. When he got back to his garage he disconnected battery so it couldnt short out in garage and start fire. When I went to pick truck up few days later, he connected battery when we started it, it backfired, and runs like crap!He has been trying to find problem for weeks to no avail!Other than pulling boat, motor has always ran like new engine till now.