TYPO3 Core

Sorry for the delay!
Of course i can give you the code, no secrets in it: http://pastebin.com/BwVcn8tv

ok, you extend from tslib_pibase, which is an alias for TYPO3\CMS\Frontend\Plugin\AbstractPlugin
Within this class, we have a lot cache logic. You also overwrite the __construct() method, which original contains cache logic/init stuff.

Did you need the t3lib_pibase class? can you try not extending the class? Or maybe call parent::__construct() in your constructor?
Also check the caching settings of t3lib_pibase.

OK, i tried first parent::__construct() within my constructor, but it changed nothing. Then i deleted the constructor und the extending of tslib_pibase. I had disable a few line because $this->pi_linkTP_keepPIvars_url is used to build the links. But in the end nothing changes, the menu is displayed only one time after deleting the system cache.

well, thank you for the tests :(

I have testet it now with a simple test class, which create entries in the menu, this works well without any cache problems.
The rendered menu is cached. I think the problem must be part of your class. it is difficult to fix a problem, if it is not possible to reproduce.

Can you reduce your class to a minimum of logic, e.g. without DB queries and so on. for example like this:

class tx_mpreferencedb_itemsProcFunc {
    public function unitsItemArrayProcFunc($menuArr,$conf){
        $count = 0;
        foreach ($menuArr as &$item) {
            $item['title'] = 'Foo ' . (time() + $count++);
        }
        return $menuArr;
    }
}

If this works, maybe you can add your old code step by step until the problem occurs again.