This is really sporatic, but has proven to be a show stopper (literally). I'm doing a show. One that I've been doing for 25 years. I'm Using the playlist and Sonar to run it. The last time this show was run was on Windows 7 and Sonar X2. This year I've installed a new SSD and windows 8.1 and X3e. Randomly, one of the songs will POP when you hit the spacebar and then play in silence. Then the next song will play fine. But it's random. I'm using files that have been used in previous versions of Sonar. I'm trying to narrow it down. Any thoughts? The show opens to the Public on Saturday.
It will happen even when not using the Playlist. I'm worried.

If you're not recording, try increasing the latency. And check out this tip. Before doing this fix, I had the problem you described.
 
Finally, are you using the Concrete Limiter, PC-2A, or VX64 plug-ins?

Yes I'm using the VX64 Vocal Strip.

Interesting. I've experienced this problem recently with the VX-64 and hadn't before.
 
Try bypassing them and seeing if the pop disappears. If it does, you can create an effects chain with Sonitus effects that does pretty much the same thing. PITA I know, but worth a try.

Thanks Craig! So far so good, sort of. Got rid of all the VX-64's (used it for de-essing some narrations). Only had one POP so far, but it could've been someone's wireless mic, because the music played through this time instead of silence. Keeping my fingers crossed.

The Blue Tubes plug-ins have a de-esser, and so does the  VC-64 if that's still installed on your system. However, I would advise against using the VC-64 if you're using a 64-bit system. Also if latency isn't an issue, consider increasing it - not a lot, just one extra "notch" (e.g., 126 to 256 or 256 to 512).