something strange... when I start the miner, it works ok, but after some difficulty adjustment from the pool (I guess it is that), I end up with a large fraction of rejected (made some adjustment -f 0.5 (to multiply the difficulty by two) it seems a bit better, but I still have large number of rejected).

The hashrate is kind of off, the reported hashrate is 1.8MHash/s but in fact I am hashing at 170MHash/s