If your arrows are tuned to your bow, the length of the feathers won'd make much difference if any. As has been said, it's more of a personal preference. I've used from 4" to 5 1/2" feathers, 3 fletch and 4 fletch, and I have found that it makes almost no difference. I've kind of settled on 5" shield cut feathers, not because it makes a difference in arrow flight, but because these old eyes can see them better.
As to why your arrows aren't flying straight, well, that's a whole other topic. Your form, torquing the string, drawing shorter/longer than you think you are, how far the bow is cut from center, and so forth are all factors in arrow flight. Your bow may not draw what it is marked either, which would make your estimate of what it draws at your draw incorrect. Have you had it checked with a bow scale? Some of my bows are right on the mark, some are off by a pound or two, and one is four pounds lighter than it is marked. If yours is like that, your 400/85 grain combo could be too stiff rather than the other way around.
In any even, I agree with Rob. Order a test kit and really figure out what is going on. If you have a veteran trad shooter in your area, see if he/she will watch you shoot and look for reasons you are having arrow flight problems.
Trying to fix this issue over the internet would be akin to a doctor prescribing drugs to someone who called on the phone and said, "I don't feel well." I might just cure them, or it might cause a whole other bunch of problems.