Braking concern on Sunfire 2000

We have a Pontiac Sunfire year 2000. My wife has been complaining "on and off" about vehicle braking problem that it slip from time to time with no
specific pattern (day or night, rain day or hot condition) . Basically she said that when she apply the brake at the stop sign or red light, sometime it would slip once causing the car to take off then stop. No ABS pump noise and no wheel lock. I have the car brake done a month ago with new rotors and new pad (old one were worn and old rotors rust a bit but still acceptable from vehicle safety inspection point of view), rear brake shoes are like new. This vehicle has ABS and traction control that always on while she is driving. I took the car out for test drive many times and could not duplicate the problem. I believe her braking diagnostic problem because she is usually never wrong about car develop new symptom that is out of the norm. My driving habit and her is not the same as she is gentle with her acceleration and stopping but that should not be a factor. Does anyone has similar braking concern as this and how did you solve it?
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I sometimes drive a 99 GMC 3/4 ton, at work, that does the same thing. My theory is that when the a/c kicks on or off, the rpm's change enough to cause the truck to move a little. Not my truck, so I haven't really investigated it but that's what it feels like. If I make the concious effort to press the pedal a little harder at stop lights, it doesn't move. H
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This problem of mine might be related. I have Pontiac Montana 2001 and I recently noticed that whenever I brake to come to a full stop, I can feel pulsating vibration like I am hitting a "regularly intervaled small bumps". I can feel it not from the brake pedal so I guess it is from something else.
Could someone give me some pointers about this problem before I go to my mechanic? I would really appreciate it.
Thanks in advance. Jasper
(PS: To reply, please remove "nospam". Thanks.)
Hairy wrote:

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Jasper 2004 wrote:

That sounds more like warped rotors. Depending on how worn and warped they are, a mechanic MIGHT be able to turn them and smooth things out. However, you may need replacements.
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Thanks for the input. Some info I got from the internet points to the same direction. Jasper
Isaiah Beard wrote:

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That is a good point! However, according to her, the AC was not ON during that time. I will check this AC cycling ON and OFF myself to see if I can duplicate the problem.

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Just a thought.
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