While loop malfunction, also center formatting is jacked up

[andyman105070]

I have this program that is almost complete, but for some reason that while loop is messing me up, I have it set so that if the 'choice' is not 1, 2 , or 3, then it should have the user re input which choice.
If a number is not (such as 0, f, 43, etc) then my cout simply runs and runs and runs.
Also, I dont underestand why "End program..." is not centered.
Thanks alot anybody!!

  // A simple C++ program
#include <iostream>
#include <string>
#include <iomanip>
using namespace std;
void center(string display); 
void showMenu();
void celsiusToFarenheit();
void farenheitToCelsius();
int choice;




int main()

{
	char choice;
	
	//Constants for the menu choice
	

	cout << fixed << showpoint << setprecision (2);
	
	do
	{
		flush(stdin);
		//Display the menu and get the user's choice.
		showMenu();
		cin >> choice;
		
		//Validate the menu selection.
	
		
		while (!(choice == '1' || choice == '2' || choice == '3'))
			
		{
			center( "Are you drunk?? thats invalid order.");
			cout<< endl;
			cin.clear();
			fflush (stdin);
			
			center ("Re-Enter your choice : ");
				
		}
		
		
		
		switch (choice)
			{
			case '1':
				cout<< endl;
				center("Enter Fahrenheit temperature for conversion to Celsius : ");
				
				farenheitToCelsius();
				break;
		
			case '2':
				cout << endl;
				center("Enter Celsius temperature for conversion to Fahrenheit : ");
				celsiusToFarenheit();
				break;
			
			case '3':
				center ("Programmer: Andrew Holder");
				cout << endl;
				center ("BYE BYE!!! Press <Enter> key to END the program...");
				
				fflush(stdin);
				cin.get();
				break;
		 	}
		
		system("CLS");
	}
		while (choice!= '3');


		cout << endl << endl;
		
		fflush(stdin);

	return 0;

} 




void showMenu()
{
	cout << endl;
center("C O N V E R S I O N");
		cout<< endl  ;
		center("=~=~=~=~=~=~=~=~=~=");
		cout<<endl << endl;
		center("1	Farenheit to Celsius");
		cout<<endl;
		center("2	Celsius to Farenheit");
		cout<<endl;
		center("3	End program.");
		cout << endl <<endl;
		center("Enter Your Choice: ");
		

}

void farenheitToCelsius()
{ 
	double Farenheit, Celsius;
cin >> Farenheit;
	Celsius = (Farenheit-32) * (5.0/9.0);
	cout << "\t\t Here you go: " << Farenheit << char(248)<<" F" << " = "  << Celsius<< char (248)<< " C"; 
	cout << endl << endl<< endl;
	center("Press <Enter> key to continue...");
	fflush stdin;
	cin.get();

		
}
			  
void celsiusToFarenheit()
	{ 
		
		double Farenheit, Celsius;
		cin >> Celsius;
				  Farenheit = ((9.0/5.0) * Celsius) + 32.0;
					 
		cout << "\t\t Here you go: " << Celsius << char(248)<<" C" << " = "  << Farenheit<< char (248)<< " F";
		cout << endl << endl << endl;
		center("Press <Enter> key to continue...");
	fflush stdin;
	cin.get();

}
void center(string display)
{
	int number = ((80 - display.length()))/ 2;
	for (int i=1; i<=number; i++)
		cout << " ";
	cout << display;
}