I have bought proxies from 3 different sellers and I am always having problems with proxies not working. Proxies work for 2-5 minutes that it says "can't connect to proxy server" I try another proxy same thing they work for 2-5 minutes then stop working. I am testing it in my web browser. None of them work for more then 10 minutes

Can this be my ISP or just bad luck with proxies I buy?

You should request for one or two test proxies before buying. Also you should contact the proxy sellers, and require proxies that will work with the site you want them to work for. I know that many proxy providers, block their proxies for sites like facebook or twitter.

I sell public proxies on fiverr and I double check them - from the first run I get ~1000 proxies and after few minutes I let it run again and get 300-400 proxies, these I give to the buyer. But still it is not possible to guarantee they will work for a long period of time.

it's more than likely user error, if you bought from 3 different providers and happens the same with all of the proxies, chances the proxies are good
it can be your ISP as you already pointed this out, your connection settings etc. hard to tell
contact one of the providers and ask for assistance, they're the pros, they can help

I boght proxies before and they were working fine without any problems, and I buy private or premium proxies. I am just wondering if ISP is doing something to my connection. They throttle my torrents so maybe it has the same effect.

Trying to figure out if ISP has the ability to mess with proxies (BTW all 3 providers use different ports)