Insane

I have some problem for my circuit below. Suppose at point A, I will get a waveform as shown in the graph below. But, instead of getting the desire results, I got a sine wave ( Vp-p = 3.XX volts ). Actually I manage to get the desire results at the beginning of my test - but after a whille the waveform change into a sinewave with smaller amplitude and remain!. What's wrong with my circuit? Do I need to make ay changes?

Hi there, I have a problem with my circuit. I suppose to get a square wave output with voltage levels 0-5V at point A, but when I test my circuit with the circuit maker simulation, it gives me a different answer. What should I need to do in order to get the desire output?