I think more information is needed to understand your problem.
How long are the part numbers? Do they only contain numbers or letters too?
Are you sure that the value list contains all of the part numbers you want?
Have you checked the field validation settings?
When you say it stops matching characters, are you still able to manually enter the entry? Does it work if you select from the dropdown instead of typing?
Usually a field stops making suggestions when you have entered something that doesn't match any entries in the value list. For example, if your value list contains 1222222 and 12333333, when you type "123" it will suggest "12333333, but when you type "1234" it will not suggest anything because there's no value list that starts with that.
Have you also verified there aren't any script triggers on the field? An OnModify script trigger could lead to a sound.