Is this just me?

700 i feel the same way. even worst. one day i am all over the books, another day it feels like i cant even add two numbers together. One day i am solving 700+ questions, another day i am stuck with simple rate, or number property problem.
Just keep pushing until it becomes your second nature. Hopefully the actual exam will be one of those "good first days".
good luck!

Yep, the same for me.

One day I can solve all problems and I don´t even need to think twice and other days I strugle with very basic questions. I´ve noticed that during work days it´s worst. During weekends I am more relaxed so I do not make so many silly mistakes.

Sure. The same as all of you. Obviously, once you have master the basics the most important factor is concentration. And indeed contration decays after having worked for a long time.

you've just described me, sometimes I get 80% right answers next day i think that I improoved my skills and hope for 90 or higher results but get 60 or 70%, and it discourages me each time. I think when we concentrate our attention on some really tough problems we loose our basic skills.

Depending on your time management, taking half a minute to clear your head can be useful. As can a "cheatsheet" written out during the test, or a better method of solving the problems(duh). 99% of the time, there's a "trick" to the problem. Sometimes taking a step back from the problem itself and identifying that trick makes all the difference. And it's quicker

One thing that you need to bear in mind is your mental state as well. Mental or physical fatigue are HUGE factors in testtaking ability. If your test center has a place to store your stuff like mine did, BRING FOOD. I myself am hypoglycemic, so this is partly specific to me. Eating a snack every break you can not only keeps your blood sugar up, but gives you something to concentrate on to clear your head.

It's tough, but try to stick to your testtaking status when you arent taking the real test as well. If you always practice when you're tired, your scores will be deflated from their baseline. This isn't going to be good for your confidence in the long run, despite the extra study time it gives. Might also want to look at placement of problems that you get wrong... If you think about it, STRINGS of right or wrong answers are more statistically unlikely than in a test with homogenously difficult problems; every problem you get wrong will be making the next problem easier, so you should not have a long string of right/wrong answers after the beginning of the test. If you've got strings of wrong answers, there is probably a correlation between problems- whether it be fatigue, or letting longer problems affect your stress level for subsequent questions.

As sad as this sounds... the biggest difference for me was food. Having a snack to concentrate on had a consistent effect on the volatility of my score.