Hmm, unsure how old is old.. But most likely you are suffering from codec issues. But even then, Kodi's software decoding should help in that department, unless you are using really slow hardware? Does the flickering still occur if you play locally stored content using Kodi V19?

Hi there
I too uses windows 7 on my old pc and run Kodi, let me share my experience.
Windows os is heavy good for mainstream application such as windows app development with plenty of resource to tap on.

However, if you will use it for daily application such a email, office and other commonly use application such as remote desktop ftp and many other applications together
with entertainment such as kodi or other iptv messaging and web browsing then you can consider installing Android into your old pc.

There are some 6 to 8 x86 os for pc which you can consider, of which i recommence you look into

1) phoenix os
2) remix
3) x86 Android

There are many more that you can read up and consider.

The advantage of duo boot android x86 and windows are. You can go back to windows as and when u need to run application that is in windows such as dedicate windows applications.
You can also boot into android and do what u do in windows with an increase in speed and support.
In android you can download most of the app from  playstore app and use,

Kodi is a very powerful application but is heavy and thus slow. consider using iptv extreme or ott navigator and many more.

If you should want to know more pm me

Read up before you proceed. The installation maybe a little complex but once you are up, you may not want to go back to windows

Good Luck