Hi Punchy. Mine started like this - all seemed well, but then it started wallowing like a tanker round corners. I checked the spokes on the rear and most were loose. I tightened those that would turn (which was about half of them) and it improved it, but when the wallowing came back I checked again and two had broken. I decided to re-spoke the wheel, as most of them were beyond adjusting, even with a proper spoke key. I now have a nice clean hub, a rim with a bit less rust than before, and a set of S/S spokes on order.
It'll be a first for me, building a motorcycle wheel, but I've done a few pushbike wheels before, so I'm pretty sure I can handle it. If your wheel is basically sound (i.e. most spokes are tight with just a few loose) then you can tighten up the loose ones until they ring. A dull, flat sound is too loose. Tap the good ones to get the idea of what it should sound like. As long as they are all pretty much the same tension, then it should be OK. If you've got a lot of them loose, try tightening them all evenly until they ring nicely - you may need to do quite a lot of adjusting, though. You can check for truth by spinning the wheel in the swingarm and holding a marker on the side - I think the service limit is 2mm out of true. More than that, and you should rectify it properly; less and you will get away with it. Don't forget to check the vertical run-out as well as the side-to-side. The rule seems to be lots of small adjustments rather than a few big ones.
The key thing is whether the nipples will turn and adjust the spoke length. If a number of the nipples are seized, then you will never true the wheel - budget for a rebuild at least.