I just bought a Y-reg Scenic 1.6 and the gearbox light keeps coming on?

I bought a Y reg Renault Scenic 1.6 petrol last Friday, and drove around the countryside on Sunday, was very happy with my purchase. Monday i drove a short distance and then noticed a warning light was lit. When I turned off the engine and turned it back on it went out, so I thought it was a fluke, but the next time the gears actually jerked and the light came back on.
When I checked - it is the gearbox light and the handbook just said gearbox problem, take it to the Renault Dealer. As this car has had 3 previous owners and I did not get it from a dealer, I checked with my regular mechanic first. He drained out the gearbox oil, which was a little low and a little tired-looking and re-filled with fresh oil, and said try it and see. At first it seemed fine, then last night the light came on again. It still drives, and we are wondering if the light could be faulty, or whether this is a sign of worse to come, and maybe I should be taking it back to where I bought it from? Help!