oric_dan:
BTW, what is the data code on your problem chips? Also, did using the full-swing oscillator
fuse setting fix the upload problem, without using the RC filter?
I have 5 x 1284P's and they are all date code 1247, which must be the latest as these were direct from Atmel/Digikey as engineering samples.
Changing to Full Swing Oscillator totally cured the problem. I'm not using any RC filters at all now and I've not had a failed serial upload since changing the Fuses. Before the changes it was failing 100% on my attempts at a 23kB sketch.