1985 Ford Tempo Starting and Acceleration Problem

Ok, I bought the car today and drove it 40 miles home and it ran great. After being home a couple hours, I took it to the store 5 miles away and it ran great, on the way home it started to shudder and was trying to stall at 55 MPH on the freeway(almost seeming like it was out of gas) and when I exited the freeway and came to a red light it did stall, but it started right back up and I made it home(trying to stall along the way still).

It starts up just fine in park now but when I put it in drive it stalls out unless i'm stepping on the brake and put the pedal half way to the floor, which also causes the headlights to dim.

once running, the gas has to be floored to get it to accelerate past about 20 MPH.

it has a new alternator, plugs, caps, wires, fuel filter and fuel lines. The belts were also just replaced.

my cousin says he believes it could be a loose serpentine belt but i'd like to know if anyone out there has experienced this.

i'd prefer to be able to pay to get it fixed instead of having to pay to get it diagnosed also

thanks for your time


P.S. I forgot to add that when it is started and shifted into drive, the RPM's drop to about half