I know Its too late to ask you, but did you find the solution?

I am setting RFM75 to power down mode by doingthe following:

CE low
Set power down bit to 0.
CE high.

then later on I power up the chip using:

CE low
Set power down bit to 1.
CE high.

then I set chip to transmit mode using your set to tx mode code example.
I send data through TX Fifo and get an interrupt back (INT0 in atmega) and I read the status register which shows 0x2E hex value. This tells me data was sent though fine.

But the receiver RFM75 on the other hand does not receive the message.

If howeever I never put the RFM75 transmitter in power down mode, the whole process of sending data works fine. The issue is only when I set RFM75 transmitter to power down and wake it up and send data.