Too many iterations yield strange results??
 

I'm a newbie and have some ridiculous problem solving conjugate problem:

Simulating cylindrical combustion chamber (species transport: air+methane) covered with cooling water case(cold water inlet + pressure outlet).

30 iterations -- result is very coarse, but physics is ok
50 iterations (fig. I) -- result looks nice
100 iterations (fig. IV) -- apparently wrong (too low temperature)
200 (fig. V) /300 (fig. VI) -- temperature variation less than 1 degree
1000/1500 -- total mess: no combustion at all, temperature variation less than 1 degree

// Figures attached