I realise it's a pain, but is it worth trying one internally on the IDE bus?

If I recall correctly, the 3400 is one of* the Powerbooks which, though capable of 32 bit Cardbus, didn't come with it enabled to the slot, and required a small hack (one or two wires added IIRC). I'm assuming that you have already done that hack? Maybe the power budget for Cardbus is higher, the existing supply isn't up to full spec, and only the Microdrives actually pull enough current to create an issue.

* (3400, 2400, Kanga?)