Battery and Stop warning lights intermittent

RS 133 Gordini 2010.
I am getting the Battery and Stop warning lights coming on for 4 seconds and then going away. When this first happened, I had the breakdown mechanic (RAC) and he found that the battery was not being charged by the alternator. Tapped the alternator (with a big metal thing) and it started charging. He recommended a new alternator. I had one installed by my Renault garage, but the problem has not done away. The new alternator is charging the battery, but their last test was to rev up the car and (for the first time) they saw the problem.
Despite this problem, I needed the car and I did a long drive (1 hour 40 mins with two stops) and while it happened once at the beginning (at slow speed), the warning only appeared when I accelerated hard back onto the motorway after a stop. The rest of the journey was OK. The only difference from normal is that I turned the fan on the A/C off.

Any suggestions?

I have now had the proper Renault alternator put in; it took several weeks to arrive. So far the problem has not come back and she has the same 'feel' as before - put a little extra pressure on the accelerator and she wants to be off (to the race track or the motorway will do). Vroom, vroom.