Fiat bravo v reg 155 hgt 2.0L, Overheat & Cutout?

Hi, I've had my fiat bravo for 6 months, and it keeps overheating after a while of driving and stopping at lights, ive had a new thermostat fitted(didnt work) and then noticed the fan doesnt cut in but i dont know why? Also every now and then it just cuts out when the revs get too low, not very often but its still annoying. Ive also had it plugged in at fiat to be told its fine. Please Help.