Problem transcoding to 120fps (used to work fine)

I've had this software for a few years and I don't use it much but when I do use it, I use it for transcoding fps.  It's always done what I've needed it to but I've had problems recently that I didn't used to have.  I'm trying to turn a 30fps video to 120fps.  In the past I did a 2x framerate transcode, and I just did it twice (although I know there is a 4x option).  But the point is, is that it worked.  And now it's not.  Transcoding to 60fps works fine, but for some reason, going from 60 to 120 is failing.  During encoding the framerate indicator never goes past ~15-18fps, and the final video comes out exactly that.  And yes, I've also tried the 4x option from the same 30fps video, and that fails too in the same way I just described.  It's been a while since I've used this software, and when I just recently attempted this initial transcode, it wasn't working at all.  It was failing every attempt.  So I uninstalled the software with Revo Uninstaller, and then reinstalled.  I paid for this software and will provide my receipt if needed.  I don't know if maybe I forgot to enable some setting or feature during my reinstall that is causing this new error now, but that's what I need help with.  I would like to figure out why I can't do what I used to be able to do. 

I'm using v4.5.210-3

I'm on a Windows 7 machine.  Before anyone decides to give me shit about that, remember that I mentioned that the few things I use this software for has always worked up until now, and I have changed nothing.  I specifically emailed support a while back to give me the installer for this version because this version worked fine for me.

edit:  So I've tried with a few different videos to see if I could figure out anything.  The main video I am trying to transcode is 4k resolution.  I've tried other 1080p videos to see if a different resolution changed anything.  I'm able to get a 120fps transcode on a 1080p video but not 4k.

edit:  ok I don't know what is going on.  I've been able to run the 4k 120fps video smoothly like 2 times out of the 30 or more times I've tried opening it up.  And I have FRAPS to confirm the fps and I slowed the video down to 50% and it was still 60fps so that tells me that the 120fps transcode seemed to work.  It's just that the 4k 60fps transcode gives me no problems at all, but the 4k 120fps transcode has some kind of issue.  I'm using VLC, but I also tried WMP to play it and it plays with the same very low fps and stuttering.

edit again:  So I've realized that this is an issue with video players and not SVP.  I looked up various remedies for lagging/stuttering video in VLC and eventually found that disabling hardware-accelerated encoding solved my problem.  It seems high-resolution, high bit-depth, and high fps videos are known to cause stuttering and lagging for many people.  It's just weird because the original video was 4k in very high bit depth and played just fine, but as soon as I did a transcode over 60fps, it all went downhill.  Anyways I'm leaving this here in case someone can benefit from it.