Caveat: I have not done what you want to do, but I think it can be done.
I am assuming that by "precision" you mean indexed shifting instead of friction shifting????
If you have a 10 speed you definitely have a freewheel and I am not aware of any 5 speed indexed shifters (not saying they don't exist). But...you can probably put a 7 speed freewheel on the wheel
http://www.sheldonbrown.com/harris/freewheels.html#7 and find an old pair of downtube or even sti 7 speed index shifters
http://cgi.ebay.com/Shimano-RSX-7-Sp...QQcmdZViewItem . Not sure about rear hub spacing but that should work as long as your deraileur has the range. If not, just get a more modern deraileur. You will probably also need a new chain, which could effect your front deraileur performance.
If you absolutely love the frame, take heart. There are plenty of guys on these forums running classic frames all decked out with the most modern componentry. The issue is always how much stuff you have to replace in order to get what you want.