VB921 troubles

[tim_s]

Hi there, newbie post so please be gentle!
I'm trying to use two vb921s to fire my 4 cyl coilpack in paired wasted spark with msns+e on a v3 board. I've managed to have the car running fine with this configuration for a few minutes, however then cyls 2 and 3 (second Vb921) stopped firing. having replaced the second VB921, the car ran fine again for a few minutes, however now 1 and 4 aren't firing. there is a slight twist in that LED17 is also not flashing now, whereas when it went wrong the first time, both LED 17 and 19 remained flashing as they should. I've replaced the first VB921 too, and still have the same results.
Spark output has been inverted and dwell control is turned on. everything appears to work fine for the few minutes its been ok, and the VB921s get warm, but not too hot. they are both adequately heatsinked. they are wired as per the diagram on the extra page, with a 330ohm resistor from the tops of R26 and R29.
anybody have any ideas as to whats going on?
thanks!
tim, BMW e30