I have solo mined for 18h: 72 stale/invalid, 16 orphans and 4 accepted.
Never gonna mine it again
I have experienced similar issues,
and, it has been happening preiodically,
When Diff of PoW is too low (due to Diff is PoS is too high ?, I am not sure),
It is going to the real time racing between all miners,
it is not a single or two orphans issues,
actually, there are so many small forks (in fact, I have seen 12 confirmed orphans also),
High Hashrate miners (such as pool or ~) are generating PoW 5~12 blocks within a minutes,
and, solo miners feel hard to get a valid chain,
Actually, QT spent the most of time to fix the local block chain,
so, even miner is working with the older/staled block only,
In fact, grain has good Diff calculation algorithm,
but, it makes some trouble(just eventually, 2~3 times a day for 1~2 hours)
Maybe, we need an effort to increase the hashrate,
it is an only way,
If you re-run QT after 10-20 minutes, it will help you sometime,