I've been playing guitar for almost a year and a half now and reached a point where I haven't been getting any better lately. To give you an idea of the level I'm at some of the harder things I can play are Creeping Death, first few riffs of Master Of Puppets and I can sort of play The Trooper. I can play a few solos too like Back In Black and Highway To Hell but nothing too complicated.

I'd like to improve in all areas of my playing but in particular my speed so i can play songs like Phantom Of The Opera, She Wolf, Raining Blood, Hit The Lights etc. so how would I go about improving my speed, both the riffs and for solos, whilst still having it sound clear (when i try to play anything quite fast now it sounds terrible).

Also, I'm starting to get into stuff like Led Zep, Van Halen and Hendrix with lots of cool little riffs and solos and stuff so how would I get better and playing that sort of music?

If you could tell me what to do or post a link to something that would help that would be great (whether its about what I mentioned or just improving in general like theory or rhythm etc.) P.S. sorry for the wall of text.
With speed, concentrate on synchronizing both hands and work with a metronome alot. Do simple exercises using each of your fingers running up and down the a certain 4 frets on each string. Also try to keep your playing clean and accurate. Record yourself playing something and listen to it to pick out any mistakes. Speed comes with practise.
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Peavey Valveking 112
practice slow and gradually speed it up.

I think this goes in guitar techniques forum btw
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Technique is perfected at low speeds not high speeds. Most people make the mistake of trying to get things to sound right at speed instead of first getting it to sound right slow. If you don't correct your fundamentals (which you can only do slowly because it forces you to focus on what you're doing) then you will inevitably hit a barrier in your progress.
Practice accuracy. Speed will follow. Play exercises as fast as you can while hitting EVERY NOTE and for the love of all that is holy use a metronome. Eventually you'll train yourself to know what something should sound like and be able to play it much faster. This is in the techniques forum in the 28 day challenge thread.
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FrustratedRocka you are a legend

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The man clearly knows his shit.

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one of the best, educated and logical posts I've ever seen on UG in the Pit. Well done good sir.
pretty much what everybody else is saying

learn some scales and arpeggios and play them slow and build up speed

use a metronome
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If you don't already, take lessons.

uhm... why?
"I'm not retreating, I'm initiating an offensive manouvre in the opposite direction!"