1998 Other Oldsmobile Models ABS & Brake System

Brakes problem
1998 Other Oldsmobile Models 6 cyl Front Wheel Drive Automatic 203K miles

1998 Olds Acheiva with 203,500 miles. Has been a sweet car to drive, no problems. Spring 2007 had rear brakes done. Off and on for several months after, ABS and Low Trac lights would come on with little/no reason. Would restart car and lights go out. Fall o 2007 car died; replaced alternator. Mechanic said brakes were "fried" from alternator slow drain, shutting down unnecessary systems to keep going, until it finally really died. (Had been slower to crank, and hadn't realized many dash lights hadn't been coming on during cranking. That's fine.) Went back and had the original shop replace the pads, which were gone after 6 months (had been in several times re: light; could never find the problem.) Guess new mechanic was right.
Since late October, occassionally have heard a slapping noise intermittently, when coming to slow stop; occasionally also would get a funny "hop" while coming to final stop. Original shop could find no problem, no codes. Took it to my new mechanic. Said pads, rest of system good. Said it must be ABS, so he "re-educated" the ABS by running it through paces with some testing/tool he has. It worked greate for a few days. Now it's worse.
Still is a very intermittent problem, but very noticeable and scary. At very low speeds/coming to a stop, I might get a "hop" out of the brakes, hear the slapping noise louder, and seems that I am using manual brakes only, but not sure. Even seemed like my car "surged" after applying brakes on freeway exit ramp at moderate speed. Today, I was braking for a turn at low speed, and the car "hopped" and I had to re-apply brakes, which then worked fine.
Mechanic says new ABS sensor runs $900. Don't want to do that with 203,500 miles, and just spend money on new alternator and lower valve gasket. This could be the start of the end for a very sweet car?
What do you think it could be? Is it the ABS? Can you run without your ABS and be safe, even with an occasional "hop" while braking? I've heard the slapping noise is an ABS noise. (Sounds like its under the engine, and a belt is slapping the pavement.)
Seems to have stumped everyone. Do you have any ideas?