Thanks for clarifying. I’ve given your question +1 so you should be able to post comments now (if you could not before). It sounds to me like this could be a bug. Copy + paste between ODPs should not exhibit this type of problem and if it does and settings do not help it needs to be reported.
The random nature might be a memory handling problem. What are your memory settings under Tools > Options… > LibreOffice > Memory ? I have “100 | 256 | 20.0 | 00:10 | 256” here. Please try adjusting your settings first and see if that makes any difference. 100 slides could contain quite a bit of information. Image handling in LO is known by the developers to have problems.
There are a few open copy/paste bugs for Impress relating to styles or specific embedded objects (e.g., Excel table) but nothing I can find that indicates your more general problem. If the memory settings do not make any difference, please raise a bug and provide as much information as possible. A test file would be ideal. The QA team will be happy to triage your problem in the bugtracker.
Please also report any bug raised back here in a comment in the form “fdo#123456”.