@Dave44 - What is the reason you stop using your sensors at 9 days? ie - Is the the question marks, the shaky data, the bad data, something else?
EDIT - I just re-read your OP and saw that sometimes you have the question marks and sometimes the bad data.
My suggestion for the question marks (particularly if right after a shower) is to carefully remove the transmitter from the sensor. Clean the surfaces with an alcohol wipe. Let them air dry a couple minutes. Carefully put the transmitter back in place being careful both of the back tabs of the transmitter click into place. (It is relatively easy to only click one tab - have to be careful and sure.)
But if you are getting bad data then it is probably just a new sensor you need.