I would have to agree with you that the setup is probably good. where it not, the problem would persist throughout the playing and would be continuous. If I am reading you correctly, the problem appears in the beginning, goes on for a bit(20-60 sec.) and then is gone and everything is fine. That argues against setup errors. My guess is that the signal is hitting something, perhaps in the receiver, that takes a bit for it to get into adjustment. Are there tubes in your receiver?
If so, one may be going out in the phono section, or may need to be cleaned. Does it happen for each record, no matter how many you play? Or is it just for the first few, say for the first half hour or so of playing, enough time for the receiver to warm up?
Have you checked the clips on the cartridge? One could be partially loose and at the furthest out point may be more stressed. Same could apply to the phono cable.
Hope this helps for ideas.
Michael
If so, one may be going out in the phono section, or may need to be cleaned. Does it happen for each record, no matter how many you play? Or is it just for the first few, say for the first half hour or so of playing, enough time for the receiver to warm up?
Have you checked the clips on the cartridge? One could be partially loose and at the furthest out point may be more stressed. Same could apply to the phono cable.
Hope this helps for ideas.
Michael