I have had the same problem with skipping on mobile streaming, but I typically blame connectivity issues (who wouldn't, with an AT&T mobile provider).
I conducted a small experiment to establish my hypothesis; perhaps you can do the same to develop a theory.
I made a playlist of songs that I had to upload to Spotify through my iTunes account on the computer that I uploaded them from (the "Host" computer). Each of these songs on Host skip. I then opened Spotify on a different computer (not tainted with iTunes) and played the same playlist - no skipping.
I had a short chat with a friend about the fact that the songs that were not previously available on Spotify (until I uploaded them) were thereafter available to me on Spotify on a different (i.e. non-iTunes) machine. Hopefully that made sense. We pondered how they could thereafter appear on Spotify without Host providing a local file which Spotify could draw upon.
I just installed this on my PC that has virtually nothing else installed (work stuff, browser, browsers, and steam. No iTunes) and it skips on every song just the way that's been described. It seems completely random when it does it. I can track back a few seconds before the point where it skipped and it usually doesn't do it again. Very annoying. I'm currently listening to the third song I have ever listened to on Spotify and it has done it on all of them.
I have never had iTunes installed on this computer. I have the premium version. It skips (like a CD, not to another track) every single song if I pick them myself (only on the desktop) and will randomly do so on songs streaming out of a playlist. It doesn't seem to matter if they are local or streamed files. I also found that occasionally you can find a "sweet spot" by randomly clicking on the slider bar for the track and it will catch and start playing, but almost never finds this on its own. I double checked my firewall exemptions and have tried it with hardware acceleration enabled and disabled with no change whatsoever.
This software is completely unusable on the desktop at this point. How practical is it that I have to have a special playlist on my phone just so I can listen to new stuff on my phone to decide if I want it in another playlist?! I love Spotify and am actively trying to get people to sign up, but this is a hard issue to work around trying to get people to join...
I had the same problem and found support to be both slow and ineffective despite being a 'Premium' member.
Here are some things I found useful, specifically for playing tracks I had saved locally.
1 - On the left side-side of your screen in 'local files'. I found if this was updating ( a circular motion couple of arrows) the skipping and client (program you're using) would become unresponsive.
2 - Deleting the cache that was part of the 'local files'.
Also increasing the cache size for me was important due to my music library. (I'm with 350,000 tracks and the max settings have been sufficient so far). [EDIT][Preferences][Cache] (I set to max)
3 - If your drive is an external drive, most of them power-down and cause issues when not spun up. I used a program that wrote and re-wrote a small text file back to the drive/s every 7 minutes to ensure it stayed spun to speed. This isn't a 'green' way to behave so I guess we should all shut down that program when we're not Spotifying.
Some forum posts talk of of deminished drive-life-durations for external drives used this way. These drives are exactly the same as usual drives: They're usually housed in egg-thin plastic to give the illusion of survivability. They might be sourced from the same, or equally, from lower quality bins. Who Knows? Just don't drop them , particularly when they're spinning their discs'. Also keep a backup.
If you need any further help, or more specific... let me know.