For long sequences of Hi continuous shooting, it is
essential to switch off all on-board processing. You do need to kill off any kind of in-camera PP incl. lens distortion correction, high ISO correction, .... Also do use AF.S or MF. (No AF.C.)
I am interested to hear that you were able to take 70 shots at 14MP: did you check that the frame rate was still 5.2 fps even at the end of the sequence ?
My own estimate suggests that you need to lower the resolution down to JPEG 6 Mp for 60 shots and more (ie over 10 sec.) without starting have a lower frame rate. I shoot myself with JPEG 6 Mp in a K-7 with a Sandisk Extreme III Class 10 card and I have been able to do over 20 sec. at 5.2 fps for the whole sequence.
If some wonders why so long sequences, there are some dynamic sequences (eg wave surfing, fountains, storms, volcanos) with high-shutter speed with which it does pay off to have some long sequences. It can also be used for some measurements in very dynamic environments, because the shots have a much higher resolution than any video movies incl. HD.