[jjoonathan]

Oh, you're in RAMP mode! Yeah, that drifts too much to hold a load calibration. This is expected behavior, it's designed to trade away precision to get speed. This will be worst on the 8350, which is completely unsynthesized, and on the upper end of each band on the 8340, which is also unsynthesized. You should see much better stability at the low end of each ramp band on the 8340 (which is synthesized) and you should be able to get that good stability everywhere if you use the 8340 source and put it in STEP mode to synthesize every point.

If you can get your hands on an 8360 source, it will synthesize both the start and end of RAMP sweeps (starting on the second sweep -- it looks to see how far off it was and corrects it), and you will also have significantly faster STEP sweeps.