any ideas on miss fire?

hi guy's
anyone help with a little miss fireing problem?
i have a 3.5 efi which runs rough at around 2800 - 3000rpm , but only when
the headlamps are on.I have replaced the battery and checked the alternator
which is producing around 13.8v. it runs perfect when lights are not on. any ideas? Thanks

oh and new coil, dizzy cap, rotor, leads and plugs

This is a new one.

Is the 13,8 volts with the headlights on?

The only other idea is some sort of earth problem. Check the main engine earth strap it may be high resistance. Try running a second earth wire to the headlights earth plus try a second earth to the dizzy case.

jiginc

Thanks for the reply. Have removed h.lamp and light boxes inner wings for welding repairs so possably an earth fault.