Could be bug or driver issue. You might try updating your drivers; 266.58 is almost a year old.

I use 2 cores to run Malariacontrol and wanted to use 2 cores for M@H but it doesn't run on 2 cores, only on 4 cores although I set the preferences on 2 cores and 50%.
Normal ?

Update : It wil run on two cores but only if no other project is running.

Found on the SETI site it uses all cores or none (or less cores but the other cores are useles). So I can't run it besides another project and it delivers not enough WU's to run day after day and also it won't start even if I set resource share to 100% and 1% at MC.

Okay let's start at the beginning...by default Boinc will NOT run on only two cores and some other project run on the other two cores when you are running multiple projects on a quad core pc. But BY DEFAULT if you set Boinc to only use 50% of your cpu's it does respect your 50% running by running using all 4 cores for 50% of the time, so it DOES use only 50% of the pc's total cpu usage, just not like you thought it might.

MOST people that use an app_info file get it from someone else and then modify it so it works for them. In this case I do not use any modification files here so I am going to step back and let Arkayn take the reins, as I have no clue about this.