dashboard lights

Good day to you all,
My desirable disco has decided that being this close to Halloween it wants to join in on all the ghoulish antics.... I switch on the lights and I have no dashboard illumination for about 1/2 a mile then they just suddenly appear..
However when I put the lights onto side lights the dashboard lights work straight away but as soon as i put them onto dipped beam the dashboard lights disappear.. curiously strange...
Apart from exhorcising the landy does any one have a starting point for me to invetsigate..?

Just recently the needle on the rev counter has become a geiger counter swinging all over the place..... more ghoulish happenings

if it helps the sensors on the alternator, ive just had mine repaired (sort of) at my local garage, although the mechanic said if his repair fails (it was corrosion on the contacts, hes apparently soldered it up or something ??) it would mean a new alternator and i may as well live with it. i think thats rubbish personally, (you must be able to get replacement sensors!) but hopefully at least thatll give you somewhere to look!!

thankyou i appreciate your help i did start to think about the alternator but that has kind of confirm a suspicion