It sounds as if the phono plug has a bad/dry joint.
I would open up the 7 pin DIN from the BeoGram, and check the soldered connections.
I have found that if I warm up the plastic sleeve on the plug, that it can be slowly pulled back to give you a better idea. Otherwise, I suggest getting a new 7-pin DIN and cutting the sleeve off, make a note of which wire is on which pin, then simply replace the plug with the new one. (Remembering to slide the sleeve onto the cable first – been there and made that error in the past !)
Hope this helps – Graham