generically, q3 uses udp (connectionless packets), but the client/server still puts the "datagrams" back into order, and the client is recieving ther packets way out of order. usually just a bad connection or whacked network tweak.
technically, certain data frames are compressed, "delta" compressed. these frames can contain events (bullet firing, death animations, doors opening). the server is always working with frames (sv_fps) that contains all the data that the client must put together. there is a buffer that the client fills up to reassemble these frames. if after the client recieves allot of new frames but didn't finish parsing all the entities it's currently workimg with, the client will start dropping the old frames (marking them as "bad", or too old) and the entity data and fill up the buffer again.
a slam of new entities in the current frame can cause this (explosions with shrapnel, a badly vis'd area, etc) combined with an unstable network configuration (bad internet connection, strange settings for maxpackets, rate, packetdup, etc) is most likely the culprit.
