(T) gearbox light keeps coming on when driving HELP!!!!!!

HELP!!!!
My fiat punto 1999 selecta transmittor(gearbox) lights keeps coming on and off when driving long distances,i have just had the automatic gearbox replaced,mechanic cannot find any reason why this is happening there are no leaks,the car runs smoothly with no noises coming from the gearbox,the only problem we have spotted is that the gear selector keeps sticking now and again,he said he will change the gear linkage cable,will this solve the problem with the transmittor light?,is there another reason why this is happening?