There seems to be two issues going on which are leading to this incorrect
behavior.
The first is that your load period is too short. You are trying to latch new
data before the old finishes shifting out. Your load time needs to be at least
16x the clock period. (8x clock for the 8 bits to shift out and doubling that
as it loads on low, outputs on high)
The second is that your clock is extremely unstable. This results in an
unstable output as that is what the output shift pulses durations are based
on.
Your load time should be at least 20-30x your
period for a safe margin due to the unstable clock. Try using a load frequency of 1k Hz and it should
function properly. You may also want to use switches rather than buttons as
holding the button can be a long process due to the simulation not being in
real time.
