XBee communication OK when sleep mode SM0 but not in sleep mode SM1


I’ve got a problem with Xbee and sleep mode.

I have a test setup with an Arduino Uno which reads an ultrasound sensor and sends the measured distance to an Xbee (S1) module.
On the receiver end I have another Xbee S1 connected using an FTDI cable to a raspberry pi and a terminal looking at the serial port.

I had everything working, so the measurement was sent reliably to the raspberry pi, but the setup uses too much power, so I started working on the power consumption. I want to put the Xbee to sleep most of the time, wake it up after a measurement has been taken and send out the result.

I used the SM1 mode, so I put the DTR (on the adafruit xbee kit) high to put the xbee to sleep, and low to wake it up. Measuring the used current during the cycle, the power consumption shows that the sleep/wake cycle works. However, data only is received only sporadically, and I receive strange characters sometimes. If I put the Xbee in SM0 (no sleep), everything works fine again (but power consumption is too high).

Any ideas?

thanks in advance