that drop is due to miner fee .... when it ends there is small iddle time .... it was written somewhere here ....
and about -i ... yes ... i have intensity 3 only on my 70s ... 80ti and 70ti with no intensity set
That's what I thought at first, too, so I decided to let hsrminer continue running for over an hour longer and the hashrate didn't change - once it dropped down to ~1150-1170 it stayed there until I restarted it.
I'm currently trying -i 5 and -c 4 as suggested by dragonmike; as of now it's been running ~50 minutes so I should have an answer in the next 10-15 minutes.
EDIT - so far the hashrate is stable after more than 1 hour of operation so once again dragonmike saves the day!
EDIT2 - still stable at around 1310.