I have been noticing something very irritating and frustrating happening when I practice guitar.

In order to improve technique, I will sit down and practice a scale/lick/whatever slowly and perfectly until I get it correct, bump the metronome up slowly. Etc etc. I will do these a little every day.

I will get better a little bit, but then start to see diminishing returns.

Soon enough, I will be putting in an hour a day into technique and start to even get worse/not see ANY improvement, so I will quit working on a technique.

3 weeks later, after playing a decent amount but NOT working that technique at all, I will notice that after warming up, I am suddenly BETTER than I was at that specific technique I practiced three weeks earlier, even thought I have not practiced it since.

Motivated again, I will start practicing that technique and start to stagnate and see diminishing returns, and even get WORSE.

I quit again, 3 weeks later come back and I am better.

What the heck? Even thought I slowly get better, the frustrating thing is that it is counterintuitive. ill practice a technique a ton, get like 2% better at it. stop for a month, come back to it and improve more than I improved while focusing on it. The problem is, if I use that logic and just not practice that technique in a focused manner at all, I will start losing it completely.
Current Gear

Bogner Uberschall
Mesa Dual Rectifier (Old Version)
Orange Closed Back 2x12 Cab with V30's
PRS Custom 24 Top 10 with 57/08 pickups
Schecter Hellraiser
Breedlove Atlas AD20
A bunch of pedals
There are several things that cause this. First, you can only concentrate on something for so long before it starts to get too monotonous for you to really care anymore.
Second, you need to take breaks so your brain and fingers can process all you want them to do.
And third, the main reason why you see improvement after stopping for a few weeks is because during that time, you're still practicing. Regardless of whether you are focusing on a technique or not, you will get better at it as you play more.
Quote by Geldin
Junior's usually at least a little terse, but he knows his stuff. I've always read his posts in a grouchy grandfather voice, a grouchy grandfather with a huge stiffy for alternate picking.
Besides that, he's right this time. As usual.
Quote by Junior#1

He said it. It's just like anything else if you overdo it, it will come back to haunt you!
Something I learned that really helped me understand why this happens -

As you're practising, the brain is creating the pathways needed to execute the motions. Practice reinforces this.

When you've had enough reinforcement, the brain "switches on" these pathways, and you "suddenly notice improvement".

Even while you feel like you're plateauing, your brain is still learning. It just hasn't "switched it on" yet. That's why you can come back to something you used to practice and be better at it. In fact I would say that's the norm, given enough practice. For me, I often find that it works like this for me...

Practice begins

3 months of minor improvement

Major improvement!

Stop practising

Moderate improvement continues for 3 months

This only applies if you're

A) Doing enough practice
B) Actually focusing on improvement during practice time

So, long story short, don't get irritated, just keep practising areas you want to improve in, the results come with time.
^ +1

And maybe, probably fatigue due to getting irritated/impatient? Just keep cool and you'll nail it.