74B-GT, Directional and Headlamp problem

Hello All, New to this site. Have a 1974, chrome bumper GT. New full wire harness, correct to specs. All clean, solid connections . Issue: Installed within past two years. Directional / Dimmer switch. Past weekend, on interstate in New Hampshire, after dark, headlights on at low beam. Activated right directional to take exit ramp. Directional works, headlamps ( both ) go out. Switched to high beams, directional works, high beam lamps come on. Disengage directional, switch to low beams, low beams come on, then later go off without directional. Switch is an aftermarket switch, either Moss or Victoria British forget which. Anyone have had this issue ?? Have owned this car since 1987. Restoration work mostly handled by well know and long experienced MGB restorer. Ideas ???