why is it that on some records, sound terrible??

Could be cartridge alignment. When I align mine, using the Linn template, I check the alignment both at both outer rim and interior to get it right.

Another common snafu is dirt/dust can accumulate on the stylus as the record plays, particularly if the record was just cleaned and is still damp and sound deteriorates accordingly until deposits are removed.

Here is the checklist that I run through in my mind whenever I install a cartridge, of things that need to be adjusted or at least considered:

Overhang.
Offset angle.
Azimuth.
VTA.
Tracking force.
Anti-skating force.
Load resistance (i.e., preamp input impedance).
Load capacitance.

It seems to me that any of the first six items on this list could account for the symptoms you are hearing, if not properly set.

Regards,
-- Al

Inner groove runs slower - it is not as good - it is not called LP = Long Play for nothing - some compromises were involved in getting LP to work. When I collected vinyl my favorites where the 12" 45 RPM - better sound - more dynamics and better channel separation.

Of course the slow groove speed at the end of an LP doesn't help, but in my experience the accumulation of dust on the stylus (as suggested by Mapman) is the culprit. It doesn't take much.

YES after playing one of the records over again and keeping the stylus nice and clean, the record sounded stable (better).