Seeing a "step" in the spectrum for the RSA306B

We recently purchased a spectrum analyzer from Tektronix. We have realized there is step in its spectrum as shown in the attached image. This step comes and goes randomly and causes unreliable measurement results.

What you are seeing is the effect of a limitation of the RSA306B. The capture bandwidth is limited to 40MHz. This causes the RSA to capture several acquisitions and stitch them together, in your case DC to 30MHz, 30MHz to 70MHz, and 70MHz to 100MHz. This is normal behavior for the RSA, and unless you are taking measurements right at the noise floor it should not interfere with your measurements.