osmocom_siggen_nogui bursty output

[the_emi_guy]

osmocom_siggen_nogui -s 12M -f 1575M --uniform --amplitude=1 -v
Generates correct signal, but it is intermittent/bursty, drops momentarily about once a second.

osmocom_siggen_nogui -s 56M -f 1575M --uniform --amplitude=1 -v
Appears to be trying to generate this wider bandwidth signal correctly, but the whole bandwidth is never there all at once, but sections of the signal are popping in out.

I have 3.6GHz Xeon, 64bit, 4 cores, 8 logical processors host.
VirtualBox set to 3 out of 8 CPU, execution cap 100%, 84GB RAM, 100GB disk, Ubuntu.
USB3.0 xHCI
No heavy apps running on host (just email, Teams etc.)
CPU utilization on host is only 25%.

Is this a processing bandwidth issue, wrong FPGA?
I'm using the hostedxA9-latest.rbf

[the_emi_guy]

Update from original poster:

New high performance PC running GNU Radio in VirtualBox, intermittent results (original post).

Upate:
Older PC, (but still high end: Dell Precision T3610). Removed Windows, running native Linux runs clean. bladerRF output is nicely continuous.