Visual Behavior Session with extreme variability in image presentation times

[alexpiet]

Describe the bug
The interval between stimulus presentations in visual behavior should be 0.75s. Some variability happens due to dropped frames. I found one session with extreme variability, that I think rises to the level of a QC failure.

Session details:
CAM2P.3
OPHYS_3_images_A
2019-04-24
Slc17a7-IRES2-Cre
V1, 375um depth
OEID: 856938751
OSID: 856295914

Here is the bad session

For comparison, here is a typical session, with some variability due to dropped frames. Note the xlimits are different between the two plots

To Reproduce

session.stimulus_presentations['diff'] = session.stimulus_presentations.start_time.diff()
plt.hist(session.stimulus_presentations['diff'], bins=100)
plt.yscale('log')
plt.axvline(0.75,'r')

Expected behavior
I expect all intervals to be within 50ms of 750ms.

Actual Behavior
We have intervals off by several hundred milliseconds.