Vibration

have a 97 subaru Impreza with 173,000 miles. Never a problem until Christmas Day 05--started shaking like it has a flat tire. No flat tire. Had tires replaced. Still happens. Switched front and rear tires. Still happens. Noticed it happens on accelleration only. Seems related problems happened since then--loss of power and stalling--Idle control valve replaced and Air intake valve--runs nice again. Still vibrates on accelleration. I am concerned that if I continue driving, the very costly repairs I just had done will need to be done again. What should I have checked? Is my car worth keeping? No one seems to have heard of this problem.
Can someone other than a dealer deal with this? Can I use used parts instead of brand new shiny parts that cost a lot of money?