Weird Lag Issues

I'm having weird lag issues with TeeWorlds:

When I run TeeWorlds, it runs normally, no lag . But later ( time changes, sometimes it takes an hour, sometimes a few minutes. ) I'm getting HUGE delay between the server/TeeWorlds and my keyboard input. Sometimes my weapon firing/changing doesn't delay, sometimes it does. My client has an Anti-Ping-Ghost feature, and when I run my ghost runs ~the size of 10 tees later, what makes it almost impossible to play. The WEIRDEST part is that my ping stays perfectly normal (20-40), even for other players! Restarting my computer or closing TeeWorlds for a minute seems to help temporarily, but it annoys me very much. Does someone know how to fix this, or at least what causes it?
Maybe it's my connection that lags out, but it never stops until I close TeeWorlds, and other things with internet run fine, even things with no internet needed.
I'm running on a slow, old Windows XP-PC that can run TeeWorlds perfectly somehow (without lag), with a normal-fast LAN-connection.