This used to happen to me. For me it was that my feet were too close together and too straight. Try widening them out and putting your feet at a 30-40 degree angle.
If you care about fixing this problem, then you should stop lifting weights that are heavy enough to compromise your form. If your knees start caving in at 120 and the reps feel difficult when doing it correctly, you might want to back up to that weight.
thanks, i'll give it another go really trying to jam my knees out and having my training partners remind me while i squat, hope i wont need to take 20kg off my squat lol