As @PeteKnight said, you need to provide more background information for people to be able to help. Things like what board you are using, the stepper motor control board type, wiring diagram, etc. would be very helpful for people to give suggestions.
From what I can gather it seems that you are trying to open/close a window via BLYNK, but also have it close if it detects rain. Please advise if this is correct, and maybe expand a bit on the description if you can.
One thing you may be able to try is to see if the timer is causing the disruption, or if it is the code within the timer function. Try commenting out the code inside the function and see if you still have the stutter. If not, slowly add in parts of the code and see where it starts to happen.
Other information that may be helpful is what is the sensor condition when the stutter happens. If its value greater than the 700 threshold that you have set in your program? Does it happen regardless of the value from the sensor?
The sensor you linked to looks like it may also have a digital output that is adjustable vie the potentiometer on the sensor board. Maybe using the digital output with an interrupt routine to trigger the closing may not cause the stuttering, as that function would only run when the sensor triggers.
You also have some of your pinmode declaration inside a while loop, is there a reason for that?
As you can see, the more information you provide the less guessing we have to do, and the easier it is for us to help. And the more likely people are to give their input. Be as detailed as you can, what have you tried and what result did it have? Help us Help you.