A weak idle mixture would cause this. When you flash the lights (or operate another high power drain item such as heated rear screen/electric windows/blowers on full etc) the alternator is put under load to produce the required current and thus the engine has to work harder.
Might be worth putting some redex through it and doing a quick service including plugs, air filter, oil and oil filter also if it is single point injection give the injector unit a clean up with carb cleaner.
The ICV should compensate quickly for the extra load and bring the idle back to 800-850rpm. If the idle is constantly low then the ICV is not working correctly and may need another clean or replacement. To be honest as long as the engine does not stall I would not worry about it for now Jeff.