Need help with the 3.7L

Hey guys, I just came across a 69 dart 4 door with the 3.7 L slant 6 all original matching numbers with 31k original miles. After getting it home and puting new tires on it I took it for a drive and it runs decent but when I get up to around 50 to 60 it falls on its face down to about 40. Does anyone have any clue on what might be causing this? I have installed new cap and rotor plugs and wires coil.
Any help will be greatly appreciated!