1993 Chevy Lumina Car shakes after travelling about 20 mile

My car starts shaking after about 20 miles. It seems to lose a little power as well. My mechanic says that it might need new plugs and cables or transmission work, but I shouldn't work on the transmission until it gets really bad. I know you can't see the car, but I recently got transferred to an office about 45 miles away and I depend on the car to get me there. I would appreciate it if you could tell me what you think. Thank you.