Personally, I've been going with the Local Record feature on Open Broadcaster Software (OBS). You have control over the bitrate and resolution you can set for your recordings, and I've never really experienced any sort of slowdown whilst recording with it. Quality is fantastic and you can even record your voice with it, which helps whenever I'm doing PC games since it can properly sync my voice up to what's happening. My computer ain't all too great at the moment specs wise, but it definitely has been my saviour recording software.
OH, and you can also record with OBS even when your streaming at the same time with it. BIG plus for me since as of late I'm trying to transition into streaming my LPs for people to watch live.
I thought I'd also comment on this because I'm having the same problem.
My thumbnails show the default gray screen with the darker gray rectangle on several of my Demolition Racer videos (which I just completed today), and despite pressing the "Regrab Thumbnail" button, I'm led to a white screen and nothing happens.
As ridiculous as it may sound, the thing that was hardest for me to swallow was just trying to play Metal Gear Solid 4.
You play through the suffering of a rapidly aging Snake, and you just feel that you want to stop and give the poor guy a rest. The fight with Ocelot was hard too because at the end of the day, you see it as two old guys who don't have much of a reason to live for anything anymore.