Hi,

When a user select an option from the size drop down list, he got a product price modification correctly. But sometimes, when the user enter to the detail product page, it display the price from the last option of size list.

Try to enter this to this URL and refresh it with F5, at least 5 times.

uniforme-cp58.webjoomla.es/preproduccion/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=6&virtuemart_category_id=3&Itemid=577

Updating price which is done by vmprices.js is nothing new and doesn't come with our VM Template.
All of these things are handled by Virtuemart itself. We don't override anything in vmprices.js
Any kind of JAVA conflict might cause the wrong price calculation, but i don't see any java conflict issue in your website.

But as you might guess, if issue happens randomly, It makes things harder to fix or point out the real problem. All i can provide you now is: use the latest version of Virtuemart and Virtuemart AIO.