Firefox Struggling to Play Streaming Video, Stuttering

HP Elitebook 9430, i7 processor, 16 GB Ram. ISP is Xfinity, quite fast.

Love this laptop. Fast, and handles everything. Until recently.

Streaming videos suddenly not smooth. On YouTube, I get bad stalling problems. It begins to play, then stops with a black screen for a fraction of a second, then continues. The actual video will play from a tenth of a second to a couple of seconds then stall on black screen again.

For Facebook embedded videos, there's no black screen, but they pause as well.

Can't think of any changes I've made for this to suddenly occur.

I'm getting 22 Mbps DL speed, and not that it matters for streaming, 6 Mbps for UL. These figures come from speedtest.net. I've checked my internet there many times, and these readings are a little slow.

Also, the stalling of streaming videos just suddenly stopped last night. I hadn't made any changes. The problem returned again today, and again, I made no changes.

I've run various virus, adware and malware checks--I'm clean.

My display adapter driver for my Intel HD Graphics 4000 are recent, and working properly.

I should also mention that using VLC to view a network stream of a YouTube video works without stuttering. It does seem to be a firefox issue.

I use Flash and UBlock. I bet UBlock might be causing it. I'll check.

I use Flash and UBlock. I bet UBlock might be causing it. I'll check. Ok, I disabled UBlock and restarted Firefox, but it is still happening. Also, I have Flash set to Ask First, so that shouldn't be enabled while on YouTube.

I cleared the DNS cache, or flushed it. Still getting the same result. I noticed a new symptom today. In attempting to watch a long YouTube video, one with multiple ads, it would play the ads consecutively and never got to the content.