Links to product pages change probably when a new session starts

[isuv]

Good evening (day, morning) to you,

We have VM 2.0.24 and Joomla 2.5.14 website. We migrated from VM 1 before and updated VM twice after that. Not long ago we discovered a problem with links. All the products belong to several categories. From time to time a product page opens at another link. Canonical link changes too. I've read here http://forum.virtuemart.net/index.php?topic=113477.0 , that canonical url is  always the LOWEST category ID url that the product is in. But in our case it seems that neither canonical nor a product url has some permanent link. What is the problem? Or who can help to solve it? The name of the website is in attachment (in order to hide from search engines). I suppose (but not sure at all) that this bug happens sometimes when a session expires. Or maybe it depends on categories, that a visitor opened before.
Note:We are not using any extra SEF components to help the SEO. Joomla SEF is on, URL redirect is on and 'Adds Suffix to URL' is enabled too in Joomla configuration. SEO suffix in VM is '-detail'.

[attachment cleanup by admin]

[isuv]

Can anybody advice anything? Because we are afraid this bug influences search engines negatively. The number of indexed pages was reduced significantly.

[Milbo]

as you said, the canonical has the lowest id of all published categories of the product. So the canonical should stay the same

[isuv]

Hi Milbo,
Unfortunately, canonical links change too. I caught this bug for you at my test website with similar configuration (set session time 1 minute). Please see the screenshots. This bug is hard to notice, as canonical links don't change with every new session, but sometimes. If you need admin account keys for the test website, I can give them to you.

[attachment cleanup by admin]

[isuv]

So, no idea what is going on and how to avoid this?

[isuv]

Product links in the sitemap, that XMAP generates, change too. Here is an example.

[attachment cleanup by admin]