Finally managed to have a good test drive today and this is what happened:
The suspect fake one,
Fully charged Lipos, run fast for 3 secs, run slow for 3 secs, run fast for 3 secs and so on < repeated.
The other one,
Always put this down to worn out Ni-Mh batteries,
That's not the case,
Car will run fine for about 7 mins, then it overheats, let it cool down and the same thing, 7 mins and cuts out.
Did the above about 4 times, Lipo still fine once speed controller and motor cooled down again.
So not very happy, Lipos are brand new.