WSPR with Yaesu FT-857D, drifting away?

Hi,

now starting WSPRing today for the first time and after a time of running all rx-reports show a drift of -1 of my signal. I think it could be a thermal effect on the trx (transmitting 2 minutes with 5 watts output could heat a little bit up inside of this nice trx).

Are there any known issues with this trx? Is the Yaesu FT-857D known for "running away" a little bit?

I take an eye on it.