1080P/23.978 FCP Capture problems.

I'm having big problems trying to capture 1080P/23.978 to FCP on my 15" MBP via an Io Express box. The same combination captures 1080i/50 with no problems at all but when I set up a project for 1080P/23.978 the capture window shows a broken image that flickers between the input and, what seems to be, the last image that I tried to capture. During this time there is no, or a very distorted, out of sync image, on the output monitors (both SDI and HDMI)

When closing the Capture window, I have to refresh the A/V devices to get the FCP image to show on the Output monitors. Playing back imported 1080P/23.978 files from FCP is just fine.

If I get out of FCP and just use the AJA Control Panel sometimes the throughput signal is fine; sometimes I get an image on the SDI out that only has part of the picture or is vertically & horizontally shifted. Again, at these times, the HDMI output has no signal. Switching backwards and forwards between "Input Passthrough" and "Test Pattern" in the Control window results in a random selection of good images on both SDI & HDMI outs; no outputs on either or broken image on SDI and nothing on HDMI. The same things happens if I play with the Genlock options.

Reset everything to 1080i/50 and everything's fine.

I suspect that my Io Express box may be having some sync issues with the 23.978 signal - but wanted to check here first just in case someone knew of a hidden checkbox that I've missed!

In case it helps, here's my system configuration:
15" MBP 2.4GHz Intel Core 2 Duo, 2GB 667MHz DDR2 SDRAM, OS 10.6.7
Io Express with 3/4 Express Card. v8.1.2 software and latest Firmware

BTW, I'm using a Blackmagic Design Decklink Extreme 3D card in a Mac Pro as my test source for both 1080P and 1080i.