Weird render problems??

I've got a fairly heavy 45 second sequence I'm trying to render and AE is behaving very strangely...

I'm using Nucleo Fast Render to do a full res QT and two things are happening - when I come back in the morning to view my render AE is saying would you like to overwrite the output module, even though there is nothing in the destination folder with even a remotely similar filename. However when I look directly into my dest folder the render had already been done - it's almost as if AE wants to render it twice. In the render queue my original render is still queued and there is no record of the one that has finished as being done!?!

AND....when I view the full res render that has actually been done it is cut short at the same point everytime I try this. It seems to coincide with the start of a new layer. Now this layer will render fine if I just render that portion around it and it will preview fine, but when rendered with the rest of the comp it seems to fall over at this point.

On face value you'd think it was a memory issue, but there have been no memory warnings that I have seen.

My comp is the following: 6720 x 784 pixels, about 70 layers - half or so are footage of various rez/PSDs/AIs and the rest are text layers.

My system specs are in my sig, but memory settings are as follows:-

max mem 120% / max ram cache 60% / secret menu: purge every 15 frames

It is annoying A) because I can't get a full-res render out of AE and B) because I can't render anything else later in the queue because after my first job is rendered (which will only happen at QTR res or so) I get this overwrite dialogue box which I obviously don't see til the next morning!!!

Any ideas guys?

AE7 Pro - Nucleo - Mocha v1.2.0 - PS CS3 - FCP 5.1.4
MacPro Quad 3GHz / ATI 1900XT / 8GB RAM / OSX 10.4.11
30" ACD / Wacom Intuos 3 A4 / XServe RAID
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Production Studio CS2 - Combustion 3 - Mocha v1.0.1
Win XP Pro 32 / Intel P4 3GHz / 2GB RAM / GeForce FX5200 / DeckLink Pro / Roland DS-5s / Sony BVM-20G1E / 2 x Dell 2007FP / Wacom Intuos 3 A4 / 110GB/80GB/600GB RAID-0