How does the TCP measurement period work?

Can somebody please explain to me how the TCP measurement period
works?  When does h_ertt decide to take a new measurement?

Motivation:
I recently saw a long-distance connection that should've been capable
of 80+ MBps suddenly drop to < 1 MBps.  Subsequent analysis of the
pcap file showed that while the typical RTT was 16.5 ms, there were a
few spikes as high as 380ms that coincided with the drop in
throughput.  The surprising part was that even though RTT returned to
a good value, the throughput stayed low for the entire remaining
transfer, which lasted 750s.  I would've expected throughput to
recover once RTT did.  My theory is that h_ertt never made a new
measurement.  However, I cannot reproduce the problem using dummynet
on a local VM.  With dummynet, as soon as I return the RTT to normal,
the throughput quickly recovers, as one would expect.

Grateful for any insights.
-Alan