940 ignition lights

I have a Volvo 940 1992, the problem is the ignition lights do not always come on. When I insert the key and turn it nothing but the seatbelt warning light comes on in the dash. The car starts OK and after driving it for maybe twenty minutes and test it, the lights usually come on. Also, sometimes the lights come on at startup, but after a while when I pull on the handbrake, nothing. This all started one year when the battery went flat, I removed the battery and charged it up, ten minutes after starting the car the handbrake light came on (the car was stationary) all worked ok until again last winter then battery died. In May this year the car started no bother, I drove to the petrol station, filled up then tried to start but no joy, the battery was totally flat. This happen yesterday on the motorway, it just stopped dead, again battery was dead. I found a few wires coming from the positive battery terminal with two fuses on them, I replaced these and the connectors as they were badly weathered, the ignition lights all came on again, but today the lights are off again.
When driving, if the ignition lights are not functioning, the wipers are slow and the electric windows are slow. I have switched on the headlights and rev'd the engine, the lights remain steady, someone told me that if they dipped and rose then it meant the battery was not charging. I totally baffled, any help would be good.