Hi Fredrick,
Ha, Skimask was thinking the same thing I was, overdriving the transmitter chip, but I guess that is not it, so . . . I would make sure the transmitter is away from the power supply a bit, and make sure to tie the grounds together all in the same place, it will help if you can tie to an earth ground. One question, are you still using serin and serout? I ask only because HERSERIN and HERSEROUT will only work in TRUE mode and not INVERTED.
HTH
JS