MacBook Pro 2011 diverse problems

I have some problems with my macbook pro for a while now, but today it got worse so now I'm looking for a solution.

I have a macbook pro 17 inch 2011, mac os x 10.7.5, 2ghz intep corr i7, 4gb 1333 mhz ddr3 memory. The motherboard has been replaced 2 years ago after a crash.

In general it is really slow. When I have too many programs open it gets stuck. I can not work in photoshop and indesign at the same time for example. When I watch a movie the screen sometimes glitches when I move the macbook. The desktop background is always blue and when you change it to a picture it turns blue again after a restart. Today suddenly the screen glitched and while i could still move the cursor, it did not react on anything. After a restart it was fine at first, than it happened again. After another restart I am now backing up my documents and for now it seems to work fine, but I really want to fix those problems now. Who knows what causes these problems?