The fact that you had the firewall and router "done professionally" is no guarantee that they are fine. Did the technician test the speed before he/she left? What was it?
They are the first thing that I would check. They are also the first thing that Comcast will point to as the problem.
By the way, I am assuming that you do not have a box with a firewall in it and a separate box with a router in it. Don't think that exists for home use.
I have never heard the term "2wire" wireless router; what is it? Just to make sure we are on the same page.
It is personal opinion, but I think Linksys is junk. They are used a LOT, but my experience with them has been terrible.
I have a client who has to replace his Linksys router about once a year. I had to replace mine four times in two months and I finally gave up on them. The D-Link products that I have used usually last for several years. Everyone has their own opinion, that is mine.
What you need to do is to bypass the router and the wireless aspect of the network. You will remove the Ethernet cable that is going from the modem and into the router and instead plug it into the Ethernet port on the computer. You will also remove the USB wireless adapter from the computer.
You will probably need to power the computer and modem off. Then power the modem on, then the computer. A very few modems require the computer to power on first, this may be trial and error.
For this test, I am assuming you have a separate modem and router. Some DSL modems have the router built in. The fact that it is wireless is not important, it should have Ethernet ports anyway.
Obviously, if it is a combination modem and router, you will not have a modem to router Ethernet cable. Also obviously, you will skip the steps that pertain to the modem to router connection; instead, you will connect the modem/router to the computer via the Ethernet cable. That bypasses the wireless part of the network.
Hopefully, you will have some Ethernet cable to use. You might have to borrow one to do this testing.
Test the speed that way. Don't use a torrent program to test speed. The nature of that beast is for the speed to go up and down constantly. Use www.speakeasy.net/speedtest instead.
Post your results and we will see where to go from there. My first guess is that it is not a DSL problem, but is just the way torrents work.
P.S. The fact that you have other computers on the network and it is happening to them is extremely important. You should have mentioned that in your original post. Same thing with the fact that it happens differently during different time periods. That changes everything.
Two things I would do, first would be to run the speed test, over and over and over during the times when it is acting bad. The other would be to run a ping, continuously for at least five minutes when it is acting bad. Since the problem happens every 30 seconds, you should see it several times during that five minutes.
To run the ping, first connect a computer that is having the problem to the network with Ethernet cables, not wireless. Do a cold reboot of that computer and the modem and any hardware firewalls, routers, hubs, switches, etc. Turn off all the software firewalls and anti-virus and anti-spyware, etc. Also turn off ALL the other computers on the network. Do not start Internet Explorer, Outlook Express, any torrent programs, etc. Anything that accesses the Internet.
Go to a DOS prompt by clicking start, all programs, accessories, command prompt. Start the ping by typing ping www.yahoo.com -t and let it run. one each line there is a time=xx section. It should be fairly similar on each line. I'm getting about 37ms on the average, but I have a fast connection. To stop the ping, hold down the Ctrl kay and press the letter c.
Are any of them hugely different than the others? The bad ones, if any, should be several times larger than the average. There should also be none of them that fail, i.e. lost packets.
To really be sure, you would need to do the exact same thing on another computer. That would greatly reduce the chance that it was the computer. It would not eliminate it because you very well might have put the same sofware that is causing the problem on the other computer.
IF that shows a consistant number during the time when you would be having the problem, it is not your Internet connection. If it shows that it IS the Internet, it still could be the modem.