Lightroom :: Not All Selected Pictures Exported

I am new to the forum, but not new to LR. I already use it since 2.2, until version 2 was discontinued I used 2.7. I was perfectly happy with it, which is why I upgraded to LR5 a while ago. Now while the functionality is great I do have quite some issues with stability of the programm.

My biggest problem is another one, and it is really annoying + frustrating:
 
After having processed all pictures I export them to JPG, for that I select the picutures and the through the export item in the menue (the usual way).
 
- LR starts processing
- LR shows progress bar
- LR finishes BUT out of let's say 100 pictures (as an example) I get always get less pictures exported, sometimes only 20% exported, somtetimes 50% etc. (at ramdom, I cannot see any rule here). If I then do the "export as previous" and skip already available picutes I get the same issue.

Since LR 5.3 the issue is really bad, as for appr. 100 pictures I need 10- 15 runs until they alre all processed.

Sometimes it works to close LR and reopen it, then the number of picutres increases ==> looks a bit like a memory issues. But hey, I do NOT get ANY message that picutures have not been exported or allike. System finishes "a normal" just when checking the result I see that I miss photos. Also: missing photos are at ramdom out of the selection (not all at end or all in the beginning).
 
My system is a Intel I7 Windows 7 (32bit) German version, with LR 5.3 in German installed, I am using a 250GB SSD for the operating system and the programms + a 1 TB HD for the pictures and LR catalogs.