If only 30 out of 1000 sound great then even though those 30 sound wonderful, as someone else suggested, your system likely sounds bright and those 30 are on the laid-back or rolled-off side of neutral.
If it were the other way around with the bulk of the recordings sounding "right" and the minority sounding poor, then I'd blame the few substandard recordings. Here, I blame the system.
It could even be one small component in the phono playback chain, but I'm pretty sure something is amiss. Phono cartridge, phono preamp, cables, and a mismatch between cartridge and arm are the most likely culprits. I'd try eliminating them one by one, maybe starting with different cables (you did mention silver-stranded cables) and then going from there. Good Luck.
If it were the other way around with the bulk of the recordings sounding "right" and the minority sounding poor, then I'd blame the few substandard recordings. Here, I blame the system.
It could even be one small component in the phono playback chain, but I'm pretty sure something is amiss. Phono cartridge, phono preamp, cables, and a mismatch between cartridge and arm are the most likely culprits. I'd try eliminating them one by one, maybe starting with different cables (you did mention silver-stranded cables) and then going from there. Good Luck.