Who doesn't recognize the situation in which a goalkeeper performs better in practice than in games or the other way round?

Either way, this has to do with intention.
And with intention I mean something like a mind set that the goalkeeper brings to the field, either at practice or at games.

Sit1: Performance in games is way ahead of performance in practice.
Most probable cause is that the goalkeeper isn't psyched up for practice. This might be a result of feeling less pressurized, rating practice at a lower value just being plain tired. Obviously there are more causes, but the solution mostly comes down to the same ;-).

By being less sharp in practice, the goalkeeper slows down his learning-curve! As an example: in stead of picking up 3 learning points at practice, he will go home with only 1 point learned. Same time invested, less outcome.

And hopefully is this awareness, that the goalkeeper diminishes his own growth-speed, enough to lead to a change in mind set.

Come to practice with the eagerness of a spunge, the willingness to learn and all the energy (& enthusiasm) you have available. Give it all you got and you'll get better at a higher rate!

Sit2: Performance in practice is above game-bound performance.
It seems fair to say that this has to do with pressure and/of fear. The fear of being judged upon (and rated "too light"), the pressure of 'having to perform'....and more. Most of these thoughts are 'homebrewed': the thought-processes take place in the minds of the goalkeepers. Obviously there's only one person that can pressurize the goalie, and that's that same goalie. So relieve yourself of the pressure.

Maybe more easily said than done ;P. In a nutshell my advice would be something like this (and this is how I deal with it myself, by the way):

I perform at my best when I'm relaxed (that doesn't mean laid back :-) ) and having fun. That means that I have to be in the here and now. The means you could apply here is focussing on your breath. Tell yourself things like: "I'm loving it between the posts", "I love the ball & the ball loves me; that's why I keep on getting 'em", "this is great; I couldn't think of anything better doing right now".

In general: Support every successfull action with positive anchor-movements (like you can see how tennisplayers encourage themselves) and ignore weaker moments (as in: evaluations are done AFTERWARDS so just let go)

Good goalkeeping & till next posting!