System specs?
OS?
Did you make any hardware/major software changes to your computer recently?
used graphics settings?
do you have antispyware?
How long has this been occurring?
anything else you can think of that could be relevant?
40-50 fps is good, and you wouldn't get any visual lag. Like apple said, Gamerail has zerio to do with fps (Frames Per Second) because it deals with you connection not any hardware like your videocard. Now, if you normally have a higher fps around 70-90 I don't know what to tell you. Try rebooting if you haven't in a while.