Why same selenium code sometime work sometime don't?

I am using the Selenium webdriver in java. When I run the code, it sometimes works as I want and sometimes error occurs. I have steady internet speed. What is the reason?

import org.openqa.selenium.By;
import org.openqa.selenium.JavascriptExecutor;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.support.ByIdOrName;
import org.openqa.selenium.support.ui.ExpectedConditions;
import org.openqa.selenium.support.ui.Select;
import org.openqa.selenium.support.ui.WebDriverWait;
public class AytomationPractiseRegistration {
public static void main(String[] args) throws InterruptedException {
System.setProperty("webdriver.chrome.driver","C:\\Selenium jar\\chromedriver.exe");
WebDriver driver=new ChromeDriver();
driver.get("http://automationpractice.com/");
By locator=By.className("login");
WebElement element=driver.findElement(locator);
element.click();
WebDriverWait wait = new WebDriverWait(driver, 5);
wait.until(ExpectedConditions.visibilityOfElementLocated(By.id("email_create")));
driver.findElement(By.id("email_create")).sendKeys("debasishgupta2015@gmail.com");
wait.until(ExpectedConditions.visibilityOfElementLocated(By.id("SubmitCreate")));
driver.findElement(By.id("SubmitCreate")).click();
Thread.sleep(5000);
String pageName=driver.findElement(By.className("page-heading")).getText();
System.out.println("Page Heading:"+pageName);
Thread.sleep(5000);
//wait.until(ExpectedConditions.visibilityOfElementLocated(By.name("id_state")));
WebElement elementState=driver.findElement(By.name("id_state"));
Select selectState=new Select(elementState);
selectState.selectByVisibleText("Georgia");
}
}

I may depend on different factors:

1. speed of the internet
2. the speed of the server you are accessing
3. the speed of the machine you are using
4. network hindrances like proxies, vpn etc.

Could be a synchronization issue. You Can avoid such issues using waits.

There are two types of waits available in WebDriver.

• Implicit wait: Its waiting time between each consecutive test step across the entire test script.
• Explicit wait is used to wait until a particular condition is met or the maximum time has elapsed.

More details available through the link: Selenium Waits