I'm also getting rpc: rejected on my 6990. tried running it for 1000 rounds+ with all rejects:

Wed Jan 01 12:17:55 2014 Worker 1 hpm 322.715261 wpm 308.805120 A/R/T 0/26/0 -> RPC result: Rejected
Wed Jan 01 12:17:57 2014 Worker 1 hpm 340.475991 wpm 307.444738 A/R/T 0/27/0 -> RPC result: Rejected
Wed Jan 01 12:17:57 2014 Worker 1 hpm 342.471463 wpm 307.474379 A/R/T 0/28/0 -> RPC result: Rejected
Wed Jan 01 12:17:59 2014 Worker 1 hpm 335.431409 wpm 309.628993 A/R/T 0/29/0 -> RPC result: Rejected
Wed Jan 01 12:17:59 2014 Worker 1 hpm 337.715411 wpm 309.957980 A/R/T 0/30/0 -> RPC result: Rejected