Are you trying to see if there were ever too many bad episodes in a row that lead to a decline in viewers? If you can't find it don't worry because the ratings for most the original episodes that aired on Fox is almost completely unreliable. They keept changing the time slots around and preempting the show most of the time at the last minute to show stupid extra football games.
(They were already showing other football games on the same days they were suppose to show Futurama earlier in the day!
There was no need to show more and "accidentally"
have no time to show the shows that were suppose to be on!) So in order to get data that would be anywhere near accurate you would have to know all the dates the show was preempted to see why there was such a decline in viewers.
For the ratings of the original episodes I would recommend looking at the ratings for when they aired on Cartoon Network. Which was more recent and they actually showed the episodes regularly. Plus Cartoon Network is also a cable channel like Comedy Central (which tend to have lower ratings than the regular broadcast channels anyway.)