Telus has a new gateway specifically designed for bonded connections, the T3200, which may help if the problem is not the line. Call Telus with a record of the times you lose connectivity and they may better be able to track down the issue. Intermittent problems are the most difficult to diagnose.
I suggest you call Telus and have them send out a Tech 2 installer to pinpoint the issue. I had a similar issue and ended up being an equipment issue 3 blocks away. The tech was monitoring the line with his test tools and once every 15 min or so is when the problem showed up, it was random and sporadic. The process was a 6 hour service call but finally found the issue. Once they replaced the defective equipment it's been fine ever since. I had Optik TV as well which made it even worse always losing connection and waiting for the Boxes to come back online.
If they've been through all the connections in your house, it will have to be an external issue. It will be the Level 2 techs job to find and fix the problem once and for all. Telus will also follow up after to make sure you are satisfied. You can try what @NFtoBC suggested first with the newT3200 first, as it's the easiest thing to try.
Make sure you get your problem escalated. The level 2 tech is more in demand and may be a week delay or more to get to you. Definitely call Telus and get a service call setup again. But make sure you state the issues you're still having. Even after a level 1 and level 2 tech have been to your home.
You will be assigned a Case ID and receive a follow up call. But make sure you're clear not to send a Level 1 Tech!
The issue is the bonded connection. I upgraded to Internet 50 (Bonded) from Internet 25 (Single Line) and thats when the problem started. I would restart the modem and the connection would work for an hour or so, then drop to unusable speeds of .5 - 1.0 mb/s until the modem is rebooted. In bonded connections, the lines need to be perfectly synced in order to work properly. If they aren't, you will notice a speed decrease as the modem accumulates errors and floods of packets keep being resent. I just had a huge escalation with Telus about this issue after being constantly told it was my switching gear or network setup. Techs would come and restart or change the modem and see things working temporarily and wouldn't even consider the fact that it could be Attenuation (KEY WORD) in the lines. It was like pulling teeth to try and get them to change the lines being used to the CO because even the smallest defect in one of the pairs can cause them to not sync properly.
My bonded connection works flawlessly.
As for attenuation etc, they can see that when they test the lines remotely. They can also see the errors too, all without even coming to the house.
Im sure most bonded connections work perfectly. Unfortunately, some of us get stuck with ones that don't. As to what they see or actually look at when they preform the tests remotely, I don't know, but Im certain they overlook many things just because their standard tests pass. In my experiences with Telus, I have found that you have to tell them how to do their job. A tech visit isn't "come and power cycle my modem, and then tell me its working". When I finally got my pairs changed at the CO everything worked perfectly. It only took 3 useless techs (all level 2), an escalation file, and a senior engineer to validate my request. Whats my point in all this? Telus isn't thorough enough in their testing and you will have to be knowledgable and tell them what you want done in order to make progress.