Why Volvo S70 dies and will not start unless I disconnect then reconnect the battery?

That sounds like you could have a problem with the car's computer. When you dis-connect the battery is gets reset.
You should get the trouble codes pulled as it is an OBD 1 vehicle. Anu Advance Auto or Auto Zone will do this for free.
Rebuilt ECUs are not prohibitavely expensive. You can use the search engine on this web site to find remanufactured ECUs from reputable dealers. Just use the search engine as you would any other and links and banners will appear along with ads for rebuilt ECUs. You might notice that even when you use any resource or write an answer or sorrespondance on this site the engine will pick up on keywords and bring up pertinant links to what you wrote or inquired about.
So, get the codes read as a bad ECU will show up.