Then, try to play the solo in bars. Like, play the first part on the first 4 beats, 1..&..2..&..3..&..4.
I remember Stairway to go in F and G for one bar, then a hole bar in A minor.
So pull out the pieces and play one part till it flows, then move one to the next. In the end, you'll be able to play it at the right speed.
Problems about speed is often, that you don't know what you're playing, therefore you start think to much, hitting wrong notes, getting off beat and actually rehearsing the wrong stuff.
So, know what you play, do it slowly and someday you'll forget about it and play just what pops to your head.
Also, as I've mentioned here before, try sing the notes you play, i.ex. sing the Stairway-solo while playing. This will develop your ear and make you much stronger, musically, in a couple of years.
