braking or trans??

when im driving and start 2 slow down...(around 15-20)...the car seems to experience a hard studder...hard enough for my head to go forward...any help would be nice...thanx

when im driving and start 2 slow down...(around 15-20)...the car seems to experience a hard studder...hard enough for my head to go forward...any help would be nice...thanx

When slowing down, when do you notice the "studder" is it:
a. slowing against the transmission (manual, auto)? No brake.
b. when you touch the brakes?
c. If it is an automatic transmission, does the 'studder' occur when it down shifts?
Any information you can provide will help to identify the problem.