I'm having this problem too. For the last several days, I set auto-test "Combined", "Express", "Every hour", "Repeat 100 times" (or even 50) and it stops testing after maybe 5 tests and sends me an email saying "Your automatic speed test is complete."
I'm on Windows 8.1, using Firefox 53.0.2 (32 bit).
Also, when I click "Force Re-Test", it opens a new browser tab and runs the test. But this results page does not have the info at the top of the page:
"This test will automatically re-run every 1 Hour. Minimize this window and come back in about 99 Hours for complete results
After 100 cycles your overall results will be shown, email reminder sent to
[email protected] when your test is complete.
Test #1 of 100 → Remember, make sure your computer does not sleep or hibernate during this process. Retesting in (56m 56s) at 11:45:03 am GMT -4.
End Test / Escape Force Re-Test wintroub's Results"
This too just started a few days ago.