I rented Mining rig #88 Dragons Den with 2.8 Mh/s but pool reports only 1.2-1.3Mh/s. It's running for more than 1 hour now so it should be pretty stable but it's not :/
Although you aren't renting my rig I have to comment on this. My rig mines at the Kh/s advertised but today when I rented it out I am getting allot more rejected than I normally see. They are all tagged
I would guess this has something to do with the way our Miners are getting forwarded to renters pools. You could try to change the server you rented to a different pool and see if that helps.
I am Rig #34 Minecraft

I'm seeing much more rejected, too. there might be too much latency between me, betarigs, then the pool.