Fibre speed variability - cause

[#1]

I have had my fibre for several days and except on the first night the BT modem has always been on. 
Most time when I have checked the line speed using the BT wholesale speed tester it has been around 73 Mbps, close to the 77.4 Mbps 'current line speed'  (from https://portal.plus.net/my.html?action=data_transfer_speed)
Now, and last night the speed drops below 30Mbps.
Should I expect it to vary or does it indicate a problem?  Clearly I had hoped 70 meant 70 - if that is what your line supports.

[#2]

Are the tests that you've been running, completed via Ethernet or Wireless?

[#3]

All ethernet (wire).
All day until now (and mostly since installed) it was steady at 73Mbps.  Now wagging around between <30 up to 50ish.  Mostly <30 Mbps.  Ping times up to just under 100ms.
Last night (~22:00) it dropped below 30 Mbps but recovered.
Tests mostly done with modem reset and browser close/reopen.
Clearly it can sustain 73.  Should I expect it to?

[#4]

Now back up to 73 Mbps.
Why would the line speed test result vary from 16 - 70 Mbps in the space of an hour or so.
I leave the BT modem on.  Should I also leave the PN technicolor router on - I switch it off when not in use and often before the BT wholesale speed test, as instructed by the test page.  Can turning the PN router on/off several times a day cause a problem.

[#5]

Hi fp,
Turning the router off shouldn't cause any problems. Switching off the Openreach Modem can cause some real problems though and will result in your line speeds being slowed as explained here: http://community.plus.net/library/browsing/fttc-dlm-what-it-is-how-it-works/
I'm glad your speeds have improved, let us know if you have any further issues.

[#6]

Here we go again - speed varying and down.
I left the PN router on all yesterday and the speed was pretty constant at around 73Mbps, with some downloads at >8MB/s.
Switched the PN router off last night, but left the BT modem on.
Tonight the speed is all over the place again and <<70.
Could someone offer an explanation what the problem might be.  Other than switching the PN router off, nothing has changed - as far as I can tell.
The exchange says the current line is 77.4 and I live about 30 yds from the green cabinet.

[#7]

By switching off the router you will have caused a change in gateway. Believe me, gateways can seriously affect your speed.

[#8]

Forgive my ignorance, but would this affect the BT wholesale speed tester too?