Yeah, my 305 has the same issue, it spikes to somewhere around 210+ in the first 5 mins of a run, and then settles back to normal. (My max is actually somewhere around 184)
It does this on a fair few runs but by no means on all of them. Im not particularly worried, as I have a Suunto, and that doesn't spike at all and reads perfectly normally for the entire run. No idea why it does it, but tend to ignore it for the first few miles of each run.