I recently purchased a 74 coupe, the tach will work for about 10 minutes and then drop to zero.  If I let the engine cool the tach will work again. Can the cable assemble be remove from the distributer body without removing the whole assemble from the block?  Has anyone experanced this problem before? 

The cable can be un-screwed from the distributer without taking the dist. out.
It sounds like you may have a bad cable end, or...the gear(s) in the dist. may be "apple cored", meaning they are worn in the center, and just will not mesh all the time.
The gear drive that the cable attaches to can also be removed, and checked. It just screws into the dist. You may want to take it out just to look at the condition of the gear, while you're there. If the gear on the adaptor is damaged, it is very likely the distributer shaft is also damaged, and will need to be replaced.
If it is just the cable that is bad, it is not that hard to replace. The hard part is getting it dis-connected from the dash/tach.
There is also a slim possibility that the cable has come loose from the tach(in the dash), and is just intermittently making contact.
A rare case would be if the tach itself is going bad. They can get dirty over time, and the dirt/grime can wear into the mechanicals of the tach, causing failures.
I'd suggest starting with checking the cable on both ends first, then the gear/adapter, and the tach last.