[popcornmix]

Code:

gpu_mem=128
gpu_mem_256=100
gpu_mem_512=128
arm_freq=950
core_freq=450
sdram_freq=450
over_voltage=6
force_turbo=0
avoid_fix_ts=0

And I'm running from a USB 3.0 memory stick.

That will set gpu_mem to 100 for a 256M Pi, and 128 for a 512M Pi.
I'd suggest you change to:
gpu_mem_256=128

if you have a 256M Pi.

Code:

vcgencmd get_mem gpu

will make it clear what is configured.