Problem with transient flow convergence
 

I'm doing a transient solution of fluid flow in a room with a radiator.

The time-step size is 1, doing 3600 steps with 100 interation each. I get to the 300th (ish) step and the solution has stopped converging.
Before hand it was converging between 20-30 iterations in.

Any idea whether this is still fine or whether something needs to be changed?