[Matt Smith]

From the pics it looks like the endfloat on the distributor is about 1/8th". Woah! It should be next to nothing, or at least is in my early dizzy. You can see the steel thrust washer just hanging out there- it should be firm against the base of the distributor. Grab the drive pinion and see how much it moves in and out.
I had the same problem a while back. The shaft had a similar ammount of endfloat as yours appears to have, causing the rotor to bash the insides of the distributor cap.
I opened it up (you need to remove the drive pinion eventually to fix this, but you can see if it's the problem by removing the other internals including base place first) and saw that the fibre washer between the internal steel thrust washers had broken down, and the distributor housing itself was being worn away internally, allowing the shaft to move up and down. It had worn down a lot, in fact exactly the ammount the 'new' endfloat was. Note, this is different from worn bronze bearings- these allow a sideways movement, not up and down.
The soloution was to remove the drive pinion (had to drill mine out eventually)and machine up an alloy spacer to compensate for the wear and reinstall a new fibre washer and steel thrust washers. You will know it's the problem when you see gunky alluminium and grease shavings under the dizzy base plate- Ouch! This is one of those examples of the fabulous flat six turning it's hand to a bit of informal lathe work on itself....

I could be wrong- I'm only looking at the pics, but if it's not then at least I hope i've drawn some attention to this particular problem!

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'72 911 TE