Servos get withdrawn from production because demand had gone below that needed to sustain production, or because better designs are available. Sometimes the servo's withdrawn because it has to be redesigned as a result of a parts maker not making certain parts anymore.

In the case of your servos, the long servo leads could be a contributing factor, or the location of the leads in relation to other leads. There could be a problem in the servo, or a problem in the receiver. Sometimes the signal driver in the receiver is a bit weak, so oscillation can develop if a too-long lead (or sometimes any lead) is attached.

Since the problem happened on different brands of receivers, the problem looks like it's in the servo when it's in a particular configuration.

The servo may need to be looked at or replaced.