I get the same. The area from 250 to 400 MHz is very bumpy and there’s a very large spike at 401 MHz. It’s probably just a limitation in the design, as all the gubbins are crammed onto one small PCB and I don’t think there is any screening between the sections. It’s worth noting that that area of the spectrum relies on an extra mix, as the tuner chip in the RSPs does not normally cover it, so the uneven noise floor and those spikes may be a result of that.
For comparison, I have a Signal Hound SA44b specan. Apparently the previous ‘A’ version had a similar problem with clumps of spikes around certain frequencies, but further work produced the ‘B’ version with better RF layout and more screening and those spikes have almost disappeared into the noise floor. Mind you, the SA44b cost about 9 times the price of the SDRPlay units!