So I figured it'll be easier to figure out certain problems if we unify our terminology; please use these correct terms for certain syndromes and we'll probably spare you another volley of forum posts consisting entirely of "but what kind of lag?!"

Lag: Not immediately visible loss of reaction time; the frame rate in the client stays the same, but interactions with other game elements (modules, chat, etc.) don't seem to happen immediately. (Note: you can observe the client frame rate by looking at the background graphics or rotating the camera around the robot.)

Stutter: Sudden drop in framerate, usually for a short period of time and often tied to other events. Afterwards, the client resumes running as before.

Slowdown or crawl: Serious drop in framerate, that doesn't go away within a few seconds.

Crash: Either Perpetuum's crash reporter dialog or e.g. Windows' "The program has encountered..." dialog appears, but there is some sort of notification that the game crashed.

Freeze or hang: No window appears, the game just becomes entirely unresponsive, often shows "Not Responding" in the title bar, mouse cursor changes into hourglass, etc.

"Just quits": The game simply quits without a sign or doesn't start.

to be expanded