My 1982 320i exhibits poor running after warming up. For the first 15 minutes, or so, it runs great (from a cold start). Then, it starts sputtering, near stalling and sometimes backfiring when under acceleration. Plugs, wires, cap and rotor button are brand new. What would cause the car to run so great cold, then begin to sputter and not accelerate after warming up? Help is appreciated, as I need to get the old girl running!