When I export a project in X3, everything plays as it should except for the drums. They play intermittently. The drop offs have no pattern. If I try to re-export the project some parts that played the first try don't play the second try. Other projects seem to be working OK. Corrupt file perhaps? If so, is it fixable or do I need to reprogram the drum parts? Very frustrating.  Or something else?  Tom

Which Drum Synth?

I had this exactish problem not with drums but tiny flatlining of the entire mix for a sample time or two. One incident per song, while exporting this one song.
 
It turned out to be the MJUC Compressor on the bass bus.
I wasn't willing to mess with what I had going so in order to carry on with it, that song has to be slow exported - audibly cause I want to.

Try freezing the synth before exporting just to muscle through it.
 
Also try fiddling with your read/write/MIDI/interface buffers.
 
aaaand if you are using Real Time Bounce disable it. If you AREN'T using Real Time Bounce enable it.
 
aaaaaand perhaps try enabling or disabling the 64 bit Double Precision Engine option.
 
Not really any solid answers there but one of those things or a combination of them might get the export done.
 
Cheers.

Sounds like a MIDI buffer too small issue. 

slartabartfast
Sounds like a MIDI buffer too small issue. 

this is exactly my first thought as well.
try bumping it up to 500.

The reason I asked "which drum synth" is that if he was running BFD2 then there's a setting on the main screen labelled "offline" which should always be checked prior to exporting, then switched back off for editing etc.

Bristol_Jonesey
The reason I asked "which drum synth" is that if he was running BFD2 then there's a setting on the main screen labelled "offline" which should always be checked prior to exporting, then switched back off for editing etc.

good point.

Yeah this caught me out many times when trying to Export or freeze anything with BFD2 in it.
 
 

BFD Eco has that as well and IIRC only needs to be used for non real time exports so my scatter gun blast of a troubleshooting post might have corrected it. Of course it's much better to just use the tool properly (by using the Offline mode as needed) and understand what's up with it. I believe there is even a special "Exporting in Sonar" section in the BFD manual (near the end). I'm guessing BFD3 has corrected this issue by detecting real time versus non real time exports/bounces and making the change automatically (dunno as BFD3 is way out of my price range and I get by pretty well with what I have anyway).
 
All moot until OP gets back to us but yeah... good call, Jonesey. I think there are similar options in other synths/drum samplers as well.
 
Just tossing in some extra (possibly useless) thoughts on the subject.
 
Cheers.