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Trans shifts to neutral at stoplight

'02 Explorer V-8 Auto trans serviced 4k ago. Shifts fine all conditions. At stop light will occasionally go into neutral,when gas applied,bumps into first. 86k on vehicle

I bet it is NOT shifting into neutral. It might not be pulling, due to low fluid, but not in neutral.

Check the automatic transmission fluid level. If the fluid level is ok, you need a transmission specialist to look at it.

I will also guess low fluid.  Check your owner's manual and it should instruct you how to check the fluid level. 

Who did the service 4K ago and why did you have it serviced?

If the fluid level is up to ‘full hot’, a possibility is that there is an air leak on the suction side of the filter. I am not positive about your transmission but a lot of filters have a pipe that slides into the suction passage to the pump. If the seal is leaking the pump will draw air. The air can be handled at speed cold but the pump may lose prime at slow speed warm.

To diagnose a little further, check to see if the fluid has a lot of entrained air after a long drive. Also, have a pressure gauge installed on the line pressure port and drive it until the ‘neutraling’ occurs and see if the pressure has dropped. If the pressure has dropped to zero, I would look for an air leak on the suction side or at least replace the filter seal and torque all the valve body bolts to spec.

Hope that helps.