Yellow Render Bar, all of a sudden.

Does anyone know why, all of a sudden, a yellow render bar is appearing when I add a simple dissolve to the timeline? I'm editing HDV and this has never happened before now. The only thing I did was to change RT from Safe to Unlimited - that's when the problem started. But I've switched back to Safe RT now and it's still happening, even after a restart. Any ideas please? I've checked Sequence settings and they're set for HDV as they've always been.

Thanks in advance.


Model Name: Mac Pro
Model Identifier: MacPro1,1
Processor Name: Dual-Core Intel Xeon
Processor Speed: 3 GHz
Number Of Processors: 2
Total Number Of Cores: 4
L2 Cache (per processor): 4 MB
Memory: 6 GB
Bus Speed: 1.33 GHz