I HAVE FOUND IT!!!
I use Windows XP (prof.) which works with proces-priorities. That way important programs can have a higher priority for the resources... the computer power.
Now I`ve set mIRC.exe to the lowest priority, everything works fine!
. Why I have this only @ this PC and not @ the others I have remaines a mystery though.
If you want to do the same. Press CTRL + ALT + DEL in WindowsXP. click on the 2nd pages "processes" (or something, I personally have a sutch windows), right click on mIRC.exe and set the priority to low.
Hmm... one thing keeps being irritating... windows doesnt save the priority. So everytime you start mIRC you have to do those actions to set the priority from the default (normal priority) to low. mIRC makers, can you help here??