"This network was wired by an amateur. Does continuity as measured by the tester, validate the wire's ability to transfer data? There could well be a kink in the cable, in a wall or the attic."
Does continuity count - no, but it is a needed start. Think of this, while voltage does need to be expressed from one end to the other, the line switches on/off with such a high rate (if I recall, 33MHz for a 100Mb/s data connection) that you're mixing getting voltage from one end to the other with an antenna to "guide" that voltage signal properly to the other end.
Rules for twisted pair as I was taught:
1) No more "pull" put onto a cable (when being pulled through walls and such) than about what a human can pull with two arms straight out in front of chest, network cable clenched in fists with fists pulling out on the cable. The cable can take "some" pull and not be damaged - but think of installation - that two arms straight out condition does not happen, rather the Tug-Of-War pull stance is typically encountered - which can easily over-pull the cable.
Why does this matter: The copper in the data/signal carrying cable gets thinned and this increases resistance between the pull point and where the cable was restricted.
2) (this is a big deal and is likely encountered - think "fitting" [cramming] cable into single gang workstation boxes - ugh) No smaller than a 6" bend RADIUS - that's a 1 foot diameter circle. Now, as a cable is being pulled from a pull-box and the cable snags - does the installer open the box to gently tend the snag or did the installer "yank it" and then see the knot in the cable - to which the knot was not cut out but rather un-knotted / un-kinked, "smoothed out so it looks good" and continue on?
Why does this matter: The four twisted pairs have specific twist rates. This is to reduce crosstalk. When a kink happens the twist rate is disrupted such that the affected portion could have greater crosstalk.
Now, at workstation boxes - which are near a network interface (router/switch/computer), crosstalk is the greatest risk as it is closest to the loud transmitter signal.
Grab four people. Put two at each end of a football field. One person at each end has a big mouth and the other has big ears. In FullDuplex mode (the typical mode for network comms), the mouth at each end is yelling to the ears at the other end at the same time. Each ears end has to hear the signal from the far end but ignore the signal from the loud mouth standing right next to it. Network cabling, aside from mixed up wiring, has two general issues: Crosstalk and Attenuation. Crosstalk is often problematic with NEXT - Near End Cross Talk (vs. FEXT far end cross talk) as the loud transmitter mouth bleeds into the ears standing right next to it.
Crosstalk happens when a transmitter's signal (the big mouth) at one end of the field jumps into the ears of the receiver standing right next to them. The receiver is unable to hear the signal from the other end of the field because the transmitter standing next to them overpowers the other end. Putting a kink or a small bend radius in network cabling near to each end of the run between electronic devices would give a greater chance of crosstalk.
Attenuation happens when the signal from the big mouth at one end of the field is weakened (that thinned copper with high resistance from 40 yard line to 40 yard line...) such that not enough hits the ears of the receiver at the other end.
A copper cable twisted pair network install is designed for 100 meters between network interfaces - a transmitter of x strength and a receiver of y capability with cable of z characteristic in between (typical as 90 meters of in-wall and 10 meters total for patch cabling for the ends). It is a designed system that needs to work cohesively. Disrupt a component and issues can happen (though, as below, it's danged robust).
While those are said, I'll also say, twisted pair cabling and the network protocols that run atop it are VERY robust. I've seen communications happen (likely with copius retries happening) over very suspect cable installs. It takes a LOT to mess up and have a non-functioning system. I'd guess your wiring is not the cause and that the computer is having issues. As it seems you have a separate system in the house, swap systems and see where the problems goes / bring the laptop to where the problem system is cabled.
network trivia question...
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