Is my implementation of a PRG at least intuitively secure?
PRNGs are a difficult and hot topic.

Some tests can be found here: What tests can I do to ensure my PRNG is working correctly?

But they do not tell you (or others) if your PRNG is really secure. A PRNG must be build in a way, that a third party is not able to "calculate" former or upcoming PRNG output based on some random data from the PRNG.