In Bulk the datarate depends on the bandwidth available with the host. There is no guaranteed throughput. If you use Isoc, you can allocate some fixed bandwidth for your image data transfer. When you configure for Isoc, the throughput depends on what is the burst/mult settings you are provided in the device configuration.
" data rate appears to be lower than with bulk": can you clarify what is your burst and mult setting and what is the throughput you get? What is the throughput you get with Bulk?
"but have issues with system crashes ": please explain what is the exact behavior which you see. Does your host crash ? What is the data rate required for your video streaming?