Hey WingNut,
Carl is always right :-) But whenever a detector is doing something odd, I always suspect batteries first. Just like you do. If the detector has a battery check circuit, and the batts check good, then they are probably not the problem. If they check bad or iffy and you know for sure that they are good, then suspect the connectors. Battery test circuits put a load on the batteries, or read the battery voltage while the detector is powered up. Even a good battery has a hard time feeding the circuit thru corroded connections. I use a cleaner by Caig Labs called DeOxit. I've seen it perform miracles before. Most any electronics shop will have a can on the repair bench. Maybe they would be willing to put a dab on the battery connections, for you. I use a small artists brush to put the tiniest film on connections. That's all you really need. And, it's too expensive to slosh it on. Radio Shack sells a small spray can of it along with another can of Pro Gold contact treatment, but it's $15 or so. Doesn't hurt to brush a bit on the coil connector pins as well.
HH,
Roger in VA