Singer Melodie 40 (5805c) - thread bunching

Question

I see you are a bit of a whiz on sewing machines - and I have just inherited a Singer Melodie 40 from freecycle. The machine is very clunky when operated and the thread on the underside of the fabric bunches up really bad. I had a similar problem with my Singer Samba and deduced you just have to be VERY careful when threading - thought this was the same so took the front casing off (as the tension wheel is inside) and re-threaded - but no - it is still bunching - any ideas????