The PIR has very high gain with a bandbass at low frequencies (0.1-10Hz or
so). Your process may cause the PIC to draw power at a rate compatible to
this and thus generate feedback with the PIR through the power supply. Try
to induce a delay of say 10 seconds between when you measure and whan you
process. This should put the eventual feedback pulse outside the band of
the PIR amp. If it helps, then you will need to apply decoupling to the
power supply and to other items (RFI from relay clicks in the same case is
one).

hope this helps,


Peter

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics