Volvo Repair: 1990 740 automatic lights, high beams, proper diagnosis

Question
our left headlight went out, the lo-beam filament, and both high beams worked.  we drove like that for a few weeks.  then we noticed that when we turned on the lights, both lights were on, even though we hadn't turned on the hi-beams,  but the lights were extremely dim.  and when we turned on the hi-beams, the engine stalled.  i replaced the original bad bulb.  that light worked, but the other one that was working previously was not working, and now when we turn on the high beams the new light works, but the engine stalls when we shut off the hi-beams.  i've now replaced both lamps, (the filaments on the second bulb i replaced were still intact), but the newest bulb is very dim and and doesn't go into hi-beam mode and the engine still stalls when you switch out of hi-beam mode.  my head is about to explode from this confusing conundrum, any help would be much appreciated.

thank you

Answer
I have never seen this before.it could be some type of loose ground wire.this is going to need to be looked at for a proper diagnosis.it will need to be fault traced with the help of a wiring diagram.I would check the battery connections bot the power and ground where they attach at the battery and engine area.