It sounds like it could be a fairly simple adjustment. One possibility is that the tracking force is set too low or it may be the anti-skating control needs adjustment. Have you checked the tip on the stylus is still present and correct?
If you upgrade you membership there is a .pdf service manual for download which will talk you through the routine adjustments. Alternatively if you can find a B&O dealer who services older Beograms they could service it for you. Our local B&O dealer charges £45 for this
Simon