No data from start_rx_streaming_at, sync_and_start_rx_streaming_at

I can’t seem to get sync_to_pps + start_rx_streaming_at or
sync_and_start_rx_streaming_at to work. The methods return true, but my
receive handler never gets called. start_rx_streaming works fine. I’ve
waited well past the time the overflow period of the 32-bit timestamp.
I’ve tried scheduling the receives 1/4, 1/2, and 1 seconds worth of ADC
samples into the future. I’ve tried it with and without an external ref
clock and pps. I’ve tried it on two different USRP2s with different
daughterboards over different ethernet NICs from different
manufacturers. I’ve occasionally had it deliver the one batch of
samples, but after I stop, retune and start again, nothing.

I’m using gnuradio 3.3.0 on Ubuntu 10.04. Both USRP2s have
u2_rev3-20100603.bin. One has txrx_xcvr_raw_eth_20100608.bin and an
xcvr2450, the other has txrx_raw_eth_20100608.bin and a DBSRX.

I’m wondering what’s going on with my setup. I’ve had problems with
transmitting and receiving at the same time on the same box, weird
timestamps when using a MIMO cable, and now problems with
sync_and_start_rx_streaming_at (See my other posts in the list archives
for more info). FWIW, I also get infrequent failures of
rx_stop_streaming and other operations.

Of course, I would never rule out bugs in my own code. If anyone is
inclined to try it out or give it a once over, I’ve posted it at in the gnuradio folder. I’d be happy to
discover I’ve done something silly.

Any and all suggestions are welcome.