Eternal Rubbing Derailleur

Is this common? My front derailleur seems to creep out of angle till the chain begins to rub more and more when I climb in small gears. It may be fine for 3 weeks and then I have to make major adjustments. It practically ruined my ride yesterday when I had to stop and get my tools out.
I just tried buying a new screw to refit the entire derailleur in case it's slipping.
But it gets worse. Often when doing a refit, I find the shift works fine on the rear derailleur but soon as I shift to the big chain ring there is one hell of a grinding noise from the front. It can take ages to adjust cable tension and angle till the noise ceases.
I have a sora at present but it gives me continual problems. I'm thinking of buying another model.
Again, is this a common problem with front derailleurs?