hi every one

I used TDS5000 series four channels digital phosphor Oscilloscopes to measure the time period of the output signal of an op.amp. switching circuit (which is varying with time, i.e. the time of each period is distinguished ) , by repeating the snapshots function, the results were far different. I feed the output of the op. amp. directly to one of the oscilloscope's input channels. the following is the electronic switch circuit. and a snapshot of the shape of the signal.

Can any one help?