I need Help please

I have a 95 disco 1 and I had to replace the engine in it all ready it has 153,000 miles on it and I got a used engine with like 60,000 on it and the thing runs like CRAP! I love the truck and Im putting a lot into it it starts fine but I start driving and it goes fine and the RPMs go down then the go back up then it back fires and spits and sputters and I can't go over 60 MPH! if I mash on the gas it shuts down and its not the plugs or wires b/c they are NEW..some days it runs like a dream and then other days I can't even make it a mile down the road. if I try to go up a hill it boggs down and wont go at all I have broken down 5 times and I tried to put in 94 octane gas and added cleaners dry gas and booster.. PLEASE help me and tell me what I can do I put to much money in it to let it go its a sweet ride I have a costom roof rack that I built and a brush guard and a DVD system and a sound system and I put a new engine in to boot and it still runs like crap....HELP p.s. when I stop at a stolight the RPMs drop and it starts to squeel the belt that is and my lights go dim!

Thanks Obie

What type engine are we talking about here?

allan

its a 3.9L gas in a 1995 Discovery V8

sounds like your running too lean on the gas try opening up the valve a couple of turns the one between the vapourizer and venturi if you are unfamiliar with lpg take it to a lpg centre to check the set up p.s. you dont say whether its any different when running on petrol or gas

I live in the US we only have gas unleaded with ethenol somtimes