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

why is it that on some records, that when you get towards the end of one side the sound of the record changes big time. were it sounds terrible, but the first few songs sounded great.

Is it the recording?
Is it the pressing process?
Is it my set up? (but not all my records to do this)

Mostly your setup. Use something like the MintLp cartridge setup tool to minimize tracking errors throughout range of travel. Some of the problem may also be pressing and recording issues.
Sounds (no pun intended) like your cartridge isn't set up properly. Different music very conceivably would be affected to differing degrees, which would explain why some records seem not to have the problem.

-- Al
I second the Mint tool. Best 100. I've spent in Analog.
well this is the problem, I just bought the new GNR album on vinyl and as the record played the sound got worse. BUT where i am confused is if i bought on a nat king cole record it plays very nice all the way through.

now with that said i am in the process of doing re-set up the TT because i haven't touched it in about 2 and years, adjusted the VTA and the Tracking Force.

I am going to run a quick comparison and i will report back soon.

Thanks for your help
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:

Offset angle.
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.

-- 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).

Another pointless upgrade avoided!