It's definitely network related. Try different servers to see if there might be a problem with the Miami location. That is the easiest solution to try first. This is also a dumb question, but make sure you are not using a powerline networking to do these tests and you are directly plugged into the modem. Try to bypass your router if you have a separate one. Again, you want to rule out anything on your end first before you contact your ISP. For instance, your ISP may just flat out tell you that you are behind a router (if it isn't provided in the modem) so you may have problem when streaming. You want to make sure you eliminate everything on your end so you can basically blame them and be correct. I think it is more than likely on their end, but just wanted to state this.
The next step is your ISP. Now, I do not know what kind of cable modem you have but you will need to log into it for this next part. If you do not, skip this paragraph. Most modems are now routers so you should have the login information. Again, I am explaining this as just for simplicity sake when you contact your ISP. You should have an SNR and a Power options if you go into the "Connection" option. I have a Ubee modem provided by Time Warner and that is how mine is labeled. The reason why I say this is because for the channels, the SNR values should be similar to each other for the most part. You also want lower power values. So, close to 0. Some will be negative and some may be like 1.0 or 0.6. Basically, if you start seeing power values in like the 3.0, 4.0, etc, that is a clear indication there is a problem. It could be with the cable wires in the house, could be outside the house, or could be elsewhere. Your modem could be bad as well.
So, lets say you find a problem, what can you do? Let's say you don't have any idea how to get to where I just described. Well, either way it will get to the same thing anyway but if you can explain the issue more clearly, customer support from your ISP might be able to better label your issue. So, if you have these spikes, it might be the lines in your house and if it is the lines the cable company provided, they are technically responsible for them. They may have also installed too many splitters causing this or some bad cabling. The ideal situation is you want one cable coming into the house (I highly doubt most cable companies would run a completely separate line for your internet) which then splits off into 2 different lines. One for internet and the other for the rest of the house's cable.
When you talk to them, you want to explain you are having issues where your internet is intermittent sometimes and you get spikes and you would like someone to check out the lines to make sure they are fine. Make sure to mention you want them to check the lines inside and outside the house. They should do this anyway but never hurts. When the tech comes there, ask them to display the power levels in the modem and ask what should be normal (it's good to know that for when you have an issue and trying to determine if it's you or something else).
TLDR: Contact your cable company and see if they can find any problems. Ask them to check the cable lines. Check for yourself to see if there are a bunch of splitters installed as well. From above, you want your internet line to be on one splitter where it branches off from your main line going into the house if they don't run 2 separate lines (rare).