@FrostLink 

Hi,

Am I understanding this right that you've had this problem with two different routers (C80 and C6) and three different computers (1x PC and 2x laptop)?

As it's quite unlikely that two different routers or three different computers have exactly the same rare problem you might want to start looking at the Gigabit switch instead. 
Can you remove the TL-SG1005D temporarily from your network or replace it with the C6 (used as a network switch) and then check whether the problem persists?

  @woozle 

Thanks that is an interesting thought. Will try this out.

But why does the network path way without switch, i.e router directly connect to the laptop also toogles between 100 & 1000 full duplex.

But still let me try your suggestion, if it works nothing like it.

Will do this next day and get back to you. It is late in the night now.

Thanks a ton !!

  @FrostLink 

I know, it doesn't make sense that the switch would cause such a behavior.

But it also doesn't make sense that you have two different routers with exact the same problem when nobody else is reporting these issues with either of the two router models.

By the way, how do you actually notice that a network link has switched from 1000 to 100 or from 100 to 1000? Are you looking at the "Ethernet Status" page of the router?

  @woozle 

Hello woozle, Good morning

I swapped out the hub for the C6 in Access Point mode.

I had the same problem, in fact the flip flop happened right in front of my eyes.

First the Laptop was 100 full duplex & PC 1000 Full Duplex, then when i tried to power cycle both routers

Laptop was 1000 full duplex & PC was 100 Full duplex.

Being utterly confused i put the input from C80 LAN port to C6 LAN port ( It was previously in WAN port )

Now all are showing as 1000 Full Duplex ( WAN & LAN )

Power cycled again, Still shows as all 1000 Full duplex which is very good.

I verify the connection speed through,

The actual speed in router status

The connection speed of the link in the computer

And the actual download speed ( fast.com or speedtest.net )  -- ( Shows less than 100mpbs or around 450 mbps while toggling )

I am not sure whats going on .. but as long as it works it works.

Thanks a lot for taking time to respond to my questions.

I will observe this over a few days to see if the toggle happens still

Thanks !!

@woozle 

Nope the problem is still there.

Going to try with a new 15 meter cable.

If this fails i will toss everything out including the ISP, cables, routers and everything and build a new network from scratch