occasionally [t]he left turn signal stop operating
The symptom description is unclear. It is possible, for example, that the exterior lamps are flashing, but the dash light or lights are not illuminating. Or you could mean that there is no audible clicking or that none of the turn signal bulbs are flashing/illuminating.
it will be a bad ground or earth as when the lights are on there is more current going through the same ground, check the ground connections and body ground points for the left side lamps, front and rear, Pat
Agreed. Your running light circuitry is clearly imposing an electrical load that is probably taxing an otherwise compromised turn signal circuit. Wouldn't be surprised if you encountered similar results - especially at idle - with the air conditioning fan on or with your foot on the brake pedal, engaging the brake lights. Although I suspect electrical resistance caused by poor connections is your problem, it is always possible, albeit improbable, that you have an under powered alternator or a maladjusted voltage regulator. I remember an E3 with an intermittent stalling issue that seemed strangely connected to electrical loads. The last thing that was checked was the alternator, with a sticker clearly indicating a rated output of 35 or 44 amps, rather than the more-typical 55A. (I suspect that someone borrowed the 35A/44A alternator from a familiar four cylinder model.) Changing the alternator fixed "that" problem, but I doubt you share the same affliction.
Tail light and turn signal connections generally do not get stronger over time, and in some cases, dissimilar metals and moisture also make for poor electrical contacts. This even includes the metal connections that are integral to the bulbs. Having said that, - this is
not a personal endorsement - but some owners of off road vehicles that are exposed to moisture extremes claim electrical connection problems are minimized or prevented with the use of something called Stabilant22 or Stabilant22A. Of course, there is always good 'ol dielectric grease on clean connections.
http://www.fixhybrid.com/stabilant-22a/