I can look at your file more later to see if there's another problem area, but until then here's some thoughts.
If your fire renders with the smoke turned off, you can render the smoke separately and composite it afterwards. Or if you can lower the particle count of the flame system and maybe increase the shape size, the blobmesh might look similar and you'll have less calculations for the keep aparts. If krakatoa is an option still, you can try that route and cache the particles out which might solve the problem.
And you can get great results with mental ray, it's just about learning how to use it correctly.
3ds Max 2009 SP1, 2010 SP1
Maya 2012
Windows 7 Professional 64-bit
Dual Intel Xeon E5520, 6 GB Ram
NVIDIA GeForce GTX 460 OC