Simple comparator not working, voltage dropped

[Vindhyachal.takniki]

1. I have made a simple single supply comparator, ckt is attached.
2. Mosfet I have used in N-channel , with Vgs = 4V. (part number is different as mentioned in ckt).
Supply voltage range is 5V - 6.5V.
3. I have to cut the output when voltage crosses above 6.5V.
4. I have tried three different comparator with LM311 as attached, OP07 & 741 also. All have same problem.

5. Problem is:

If difference at Vin- & Vin+ is high then opamp/comarator works fine i.e full swing at output, which turn off/on mosfet completely & circuit is switched off/on accordingy.

But when voltage difference at vin- & vin+ is very low or almost equal, output voltage of opamp falls. From 6.5V it remain 4.5V or nearby. Dut to mosfet don't get on/off properly & drop across mosfet occurs.. I don 't understand why this problem.

[Zero999]

There's no hysteresis. Try adding a 1M resistor between the comparator's output and the non-inverting input.

[Seekonk]

I bet it is probably oscillating too on any noise.  Your lower voltage is probably the average of that.  Feedback will solve that.