i have a really annoying lag in my game

hi,i have this really weird and annoying lag in my game,especially in the last few days.i will try to explain how it looks like.so in the middle of the game i lose control over the game and my character starts walking in all directions,walking into walls and shooting around until it spends all bullets.this lag lasts only a few seconds and it happens 2-3 times a game,but its really annoying because i usually get killed during this.also when i am in the lobby i sometimes lose control over my mouse and it starts jumping all over the screen.this already happened a year ago and people told me that hackers have hacked into my computer and that they are trying to control me while im playing.then i scanned my computer for viruses,defragmented my disk,cleaned my registry,reinstalled crossfire,and i repeated this 5 times and then i finally got rid of this.i don't wanna do this again so can somebody please tell me how to fix this.

p.s. this might be connected with avast because it scans crossfire all the time and sometimes i have to reinstall crossfire because avast deleted some files from the crossfire directory in program files(it usually detects a file called rootkit.exe or something like that as a virus)