It's a fairly common problem on the Grundig 4870, I bought the circuit diagrams and service manual for it spending some time on the problem without success

The problem is, when you take it out and set it up on the bench you have to wait considerable time for the fault to occur and as soon as you start looking it dissapears

I would love to know the answer and somebody somewhere knows it, but they 'aint published it

I replaced mine with a Grundig 4871 from German ebay, which after sorting out various other problems is still working ok.
It's an awesome radio and cd system when it's all working, but best replaced with a good modern setup if the problems begin to get too irritating
