hquezada,
You did not mention which RBW shape/window function you are using. I would recommend the Nutall window for this measurement. It will speed up the measurement by about 2x which will help with stability.
Based on the sweep time and warning you are receiving, it sounds like the PC is unable to keep up with the measurement. The measurement as configured will require high sustained USB throughput and high CPU usage to perform, more so than sweeps with larger RBWs, which is why you are seeing issues at this RBW and maybe not others.
The 250s sweep time tells me the API is likely falling behind and getting in an undesirable state. I have configured this sweep in Spike and get consistent ~6 second sweep times. Are you able to reproduce this in Spike?
What is the make/model/CPU of the PC you are using?
Regards