Riva:
The transmitter will probably generate a random number when first powered on (or batteries changed) and this will become all/part of the preamble.
I'm not sure about this, because - how its possible to sensor to generate the same number again when putting back 'normal' batteries?
As you can see from here (and I don't know if I cut the lead 'zeroes' properly) - we have the same preamble after tries with 3 other sets of batteries.
Compare: "starting point" and "typical setup" - preamble is the same.
0001010111 01010011010001011101010110011000 - your preamble (7,1, RH93%)
0010100100 00011100010001100000001000000001 - starting point
0000101111 10010010001101100111001000001010 - battery + old ni-mh
0010101101 00011100001101100101001010001010 - used battery (1 GOOD, 1 'used')
0011110101 00010111010001100100001010001011 - used battery (1 GOOD, 1 nimh used)
0010100100 00011100010001100011001010000110 - typical setup (2AA varta batt.) (same as "starting point")
Great job with the temperature - as I see the biggest problem will be the code. 