Lets try to narrow down this issue.
Check for hash tables.
//var %x $hget(0) | while %x { echo -ag $hget(%x) $hget(%x,0).item - $hget(%x).size | dec %x }
Uptime.
//echo -ag mIRC: $uptime(mirc,2) - Computer: $uptime(system,2)
Tools -> Options -> Other -> Windows buffer: ???? lines.
What is the number?