Ethernet port on 2010 iMac won't operate at Gigabit speed, but requires router to be set at 100 Base-T.

Question

For almost a month I have been experiencing internet connection dropouts with my vDSL line, which had previously been stable for about six weeks since I changed ISP. I have an initially wired setup, from the router to my Time Capsule WAN port and from there to my 27" iMac Ethernet port. I bought new Cat 6 cables on setup.
During the router setup I was advised to set the active port at the Gigabit setting. Finally having discounted all other possibilities, the router manufacturer support engineer suggested I reset this back to the default 100 Base-T setting: and this has resulted in a stable connection once more.
Surely my iMac should offer functionality at Gigabit Ethernet transfer speeds? Otherwise the Airport home network is dependent on what my computer's Ethernet port can manage.

Answer 1

If you Google for non Flash speed test site you will find at least two, I have no idea how good they are hence not putting any links in the post.
As for pin pointing the problem, the only way I know short of having access to the internals of the router and TC including firmware source and test equipment is by producing the fault and swopping the various pieces of equipment out. Having access to the internals is way beyond anything I can do and swopping the parts out seems expensive.
From past experoence of Gigabit issuses I had it was usually down to cabling or faulty connections in the RJ45  sockets, this was quite common. As for the incompatibilty, one way to test is to put a 1Gb switch between the router and the TC.
I would not presume either the router or cable are good until proved.
There is always the possiblity that more than one piece of equipment is faulty, rare but I I have had it happen before.
Not a lot else I can say, perhaps someone else as more thoughts and can help.