Weird lag that makes my keyboard input get a delay but my mouse input is fine

Hey guys, I've already submitted a ticked but they routed me to a hextechnichan and he kept saying the same thing basically I couldn't make him understand my problem. My issue is a weird kind of lag, it only happens ONCE in a game, but it happens every game. Between 5-10 minutes i suddenly notice that the sound is behind (Like when i die it takes a while to hear my champs death sound, or the announcer lags behind) and my abilities, my camera lock, (space bar) my settings, my chat starts to lag like crazy (unplayable levels of crazy) but anything i press meanwhile im in this state of lag (which only lasts about a minute or so) gets queued, so if i type anything it does indeed type it out but with a weird kind of FPS lag, meanwhile my FPS is stable 70-80 which it should be on my setup (I've always had around this fps) I don't notice any kind of ping or FPS spikes. But whats really makes this weird that anything i do on the keyboard lags meanwhile im in the lag mode (don't know how to call it at this point) but my mouse input is unaffected by my lag. Never had this kind of issue before and it began like 2-3 weeks ago. If any of you guys had a similar issue and know a fix please tell me about it so i can test it.

I've tried:

• Closing the client while in game ( in the options, not manually)
• Windowed mode ( It does help, however my potato PC can only run the game at the fps im confortalbe with on fullscreen)
• Setting the video options to the lowest

And some additional info: Once the lag was intense, so i used alt tab to let my GPU catch its breath the game put itself in windowed mode for some reason when i got back. (only happened once) Also if i use alt tab while im in the lag mode it does the same and gets in a queue, but if i do it the problem gets less intense and goes away quicker.

Hope you guys can help me!