Reduction prices are not shown correctly on product page

[Jaroslav Vinduska]

I have prepared price reductions for several products on my shop (www.led-svietidlo.sk).

When I open list of Specials (price drops), all products are correctly shown with reductions by prices-drop.php. When I click on a product, it is open by product.php.
At the first moment the price of the product is correctly shown considering price reduction of the product. After the complete product details are open at the screen, prices change to standard product prices (retail prices excluding reductions). If I use Ctrl+R to refresh the page, correct price is being shown for a while until complete script is open, then it changes to incorrect prices (retail prices without reduction).

This only happens with some products. It is unbelievable that after some time the list of products with wrong prices displayed changes.

Does anybody have the experience with a solution of this problem? Please, help me.

Thank you very much.