Question about sporatic downbload speed.

I have 2 SSDs. The one with the OS has 94Gig free. The other I use for storage and has 105Gig free.
The computer has a Intel i7-4820 processor and 16 gig of ram.

I will change those DNS addresses in the AM.
I know Comcast uses their own DNS of 75.75.75.75 and make that default.
As far as throttling, Comcast supplies a tool to see if your being throttled, but it's a Comcast supplied tool so who really knows. It shows I have had no throttling.
Thanks for your input and I will let you know if things get back to normal with the DNS change.

I changed the DNS in both my router and computer at 6:00am.
Should know if that helped anything by about 6 or 7 this evening.

Changing the DNS didn't help at all. Speeds are starting to go down and will do so until I re-boot the computer.
Actually the 8.8.8.8 has slower page openings than Comcast's 75.75.75.75 default DNS, so I will switch back.
I asked 4 of my neighbors who have Comcast if they notice any slow downs. They don't have a any problems, but all of them are on a lower tier not the extreme.

was solved when the line was checked outside and discovered a wasp nest was in a junction box, grounding the signal out , was worst during wet weather.

My first thought was the ISP, but every wire to their tap has been changed and a new splitter.
They tested signals. ingress etc and all comes out good.
The thing that is really puzzling is why I only have to re-boot my computer and not the modem to get the speeds back? Even then it's only for about 12 hours. They also tested congestion on the node and it's only at about half it's capacity. This is what Comcast told me anyway.
Other than the NIC, which I have changed, what in a computer can cause this very weird problem?

That's a sign it's your PC. Then again you might be running torrents. That discussion has been done so many times. Take that back to the torrent folk please.

About the only thing I do out of the ordinary is Slingbox Seattle Seahawks game to my son in California during football season.
I have redone my routers SSD and passwords to where no one could figure that out, plus I have Netgear Genie which shows me everyone and everything connected to the system. No one that shouldn't be on there is.
What can cause speed issues on a computer other than the NIC?

To figure this out I would have to see all details. Remember we are not seeing the ISP slowing down, just your PC. I take it you are only seeing it on the PC so what's with this PC? McAfee?

Other than the NIC, which I have changed, what in a computer can cause this very weird problem?

Fluctuating voltage levels from a failing PSU, or ripple caused by alternating current bleeding through from failing motherboard capacitors. A number of things can cause it. Any other problems that might offer added clues? Wave action on the monitor? USB device that maybe quit working recently?

Didn't think about things like that causing a speed problem.
I really don't have a way to check voltage from the power supply. It's a 850 Watt supply with only one graphics card, two SSDs and two Blu-ray burners so don't think there is a lack of power.
I use a UPS that is at a pretty constant 119 volts out on all 8 plugs.
I think there are programs out there that looks at the power supply and see what's being used, just don't know which are good and the ones that are not.
The computer is about due for it's cleaning so will look close inside when I do that.
I think I will just due the re-boot for now.
I have wasted about 6 weeks on this problem with no solution in sight.

Thank you Bob and James, but just one more question.
Had a friends son who is a tech at a computer shop came over with his testing gear. He said the power supply is right were it should be.
He sees no hardware problems at all after testing and looking at the computer.
He also said that the computer is very fast on everything except the internet when the speeds start going in the tank. All tests were done with no router and going straight to the modem. Also changed patch cords to both cat 6 and cat 7 at different times.
Unless either of you have any suggestions, I will call Comcast one more time. If they, again can't find anything wrong, I will just bag it and keep re-booting every 12 hours.

most common cause without any inside hardware problem would be something grounding out the signal.

This whole problem is really a weird one, but everything points to my computer. That computer tech said he hasn't run across one like this before.
My computer is an Alienware Aurora R4 that is still under warranty.
Before Dell bought Alienware, they were built with top of the line parts, not now, Dell uses OEM parts as cheap as they can find, so it could very well be a computer problem. If it is the computer, Dell will be repairing it.
I have been waiting until Windows 10 comes out to buy a laptop so will get one next week, then test them side by side just to prove one way or the other.
Thanks for your input and will get back in a couple weeks.