iPods can't play many songs bought from iTunes store 3-4 years ago

I have a iPod problem with playing songs from iTunes store 3-4 years ago. First of all, it is a very inconsistent issue. Even in the same album, some songs are ok. But, problematic songs always stop and skip at the same time position (move to the next track). Moreover, it happens some songs in multiple different albums. In some albums, most of tracks behave like this. In other case, only one track in the album has this problem. I have probably 200 albums from iTunes store. I found this problem in at least 10 albums. Interestingly, this kind of songs increase over the time. About 2 years ago, I noticed only a few. But, I currently have 100 tracks like this... I have never experienced this kind issue in other MP3 I bought from other sites (such as Amxzon).

It happens in different iPods (two generations of 160GB classic, 4th generation of iPod touch, and an old 30GB iPod). It also happens songs from the different Macintosh computers and various version of iTunes (iMac, MacBook, G5). In other words, it seems the problem is in the song tracks.

I analyzed those bad song tracks using Audacity. It looks the track is intact, and no apparent glitch in the track. Moreover, they are fine after I converted to MP3 files.

My problem is that I have two many MP4 files like this (Probably 100 files). I want to keep the sound quality. Most important, they have to be played in any iPods.

Any thoughts and ideas are welcome.