I have had the same problem in the past. Exactly. Pretty happy up to the 7 iron and then blammo. Getting a nickel's worth of difference from 7 down.
Back in June I took a lesson from a decent pro who had me get on a machine and take swings. It is set up with two cameras giving me a front and target line view real time. First she had me take my normal swing a few times and then showed me a videotape. I have seen enough good swings on TV and in print to immediately spot at least three areas of my swing that were flawed and I had no idea I was doing those things. They were:
1) Overrotating my hips to get a fuller backswing
2) "breaking" my wrist in order to get back farther
3) Going past parallel in backswing.
4) Going too far "in to out" resulting in low hooks especially with woods and drivers.

In addition to inconsistent results I was wearing myself out trying to get distance. This swing was built myself from practice trying to get rid of a slice, which I accomplished. Unfortunately I went too far the other way.
Basically I had to rebuild my swing. The Pro's input was a help but in retrospect most advice was a band-aid solution. She had me weaken my grip and suggested thicker grips for instance. Not that there's anything wrong with that but they really were band aids.
The best thing was she allowed me to drop in anytime and use the video stuff when it wasn't otherwise being used. I cannot estimate enough how great a help that was.
Another big help to me was Dean Rheinmuth's "Tension Free Golf" book. I could go on about it but it just helped my finalize the lessons I kept relearning about letting the club do the work and not trying to muscle it. I think some people could read it and it would do almost no good but for those who really pay attention and internalize the message it is very worthwhile.
Lastly and most important I had the will to incorporate those changes and give those lessons time to take hold. Anytime you make swing changes it feels weird and awkward. Unfortunately sometimes you just have to do it and accept that until you've done it enough times to feel natural it can take months, as it did for me. I am glad I did it. Oh, another thing about equipment get rid of long irons altogether. I am using a 7 w instead and like that a lot better than messing about with the 3i. I don't claim to be a mindreader and don't want to offend but sometimes other people's input on your swing means squat. When my swing was even more hideously flawed than it was later I asked a friend who was good for input about my flaws. He must've known about 100 things I was doing wrong but he wouldn't say a thing. Long story short feeling than your swing looks decent and having others say so only means something if you're hitting good shots most of the time. The proof is in the pudding. If you are getting these results there is a flaw and if you really want to get better you need to find it and fix it.