Press F11 and bring the up status bar, it has 4 different colours, Blue = Latency (300 is about average), Green = Graphics over 12-13 FPS will generally be ok in combat, Yellow some thing to do with server lag, if its high 50k or something it usually means its a SOE problem (zone lag etc) and i think the last one is packets lost which is red and should be 0 if your loosing loads of packets could be an issue also. When this happens again just cycle through the colours by clicking them and record what they are.
Green - Upgrade Graphics Card / Drivers
Blue - Your internetz
Yellow - SoE problem
Red - no idea but id check your connection to internet if its wireless make sure you have a good clear view from wrouter to your wireless card