First of all,I want to say that English is a foreign language for me so I may do some errors.
Secondly, I want to thank all who even bother to help me to solve my problem.
And now, THE PROBLEM:
My internet usually works well after turning the computer on,but after some time (5-10 mins, sometimes more, sometimes not at all) it slows down A LOT or even cuts the connection off completely.I surf net very often and also i like playing online games so this is very annoying. When i play a game my ping in one moment is 50-100 but in another it reaches 1000,therefore the game is unplayable with this much lag. After some time(10-20 mins,sometimes more sometimes)the problem fixes itself but after another 10 mins it starts from the begining.It also seems to fix sometimes when i run network diagnostics but in most cases this doesnt work.Also the diagnostics show different things each time i run them: ,,no error has been found'' or ,,default gate is unavaiable" or ,,server DNS is unavaiable".
Whats more,my Firefox is recently having another issue. This happens each time i change pages in my browser and then my AVAST pops out with a message that he blocked a dangerous site, and the infection is supposed to be ,,URL:MAL" whatever that means. This doesnt seem to have any impact on browsing but I've found somewhere on the internet that this might be a virus. I ran some scanning and found some malware but after erasing it both of those issues still persist.
Also it once happened that AVAST said that it found a rootkit. So i let it to scan the entire computer, but i dont know if the rootkit was erased or not,but there were no more pop-ups about it.
I am using Win 7 x64 as system, accessing my router trough TP-LINK wireless adapter model TL-WN821N, the router is NETIASPOT 5E66F0 with the newest drivers.
What should I do? But pls be patient as I dont know everything about computers and try to explain everything to me step-by-step.
Your english is perfect. No problem at all.
You can try to disable IP V6 (read here) on your network interface. It helped here for a problem where I had small network disconnections. It may help you too.