Disaster...!!!

I think my overlocker is broken... or maybe I am doing something wrong?? I have only had this wonderful machine for a few months and so far it has been very good to me. Until now...

It works for a few inches if I only have a single layer of fabric and then the thread breaks. If I put 2 or 3 layers (of normal weight cotton) it breaks after an inch or so, and it also skips stitches.

Here are some points that might be useful:

1. I am using the same thread as I have used in the past (it was supplied to me together with the machine)
2. I have replaced the needle
3. I have re-threaded the machine (more than once!)
   
4. I am using the same type of seam as before
5. I have tried changing the tension on the various threads

So far no luck. All ideas of what to try next are welcome. :o(

Update:
I spoke to Sewing Machines Direct and explained my problem. They were very helpful and sent me 4 cones of good quality thread (I had been using the thread that was included when I bought the machine). They thread arrived the day after I spoke to them and now my machine is back to normal!!! Sooooo happy!!