var myval = Number(document.myform.mytext.value);

//var myval = document.myform.myval.value; myval++; myval--;

var myval = Number(document.myform.myval.value);

//var myval = document.myform.myval.value; myval++; myval--;

var myval = document.myform.myval.value; myval++; myval--;

Javascript sometimes confuses strings and numbers.  When this happens, you get bizarre outcomes such as "2 + 2 = 22".  The following code FORCES a conversion to a number.

Convert String to Number in Javascript

JavaScript