1994 Jaguar XJ6 Won't start at times

With key off, remove negative cable from battery. Check for 12 volt, full voltage, draw between the negative battery post and the negative battery cable with an incandescent test light. If light is on "full bright", start pulling fuses until test light goes dim keeping in order so they go back in the same space. The circuit that makes the light go dim is the one that is killing your battery. Then, reinstall the fuses one at a time. If the test light goes back to and stays bright, you've found another full draw circuit to check out.