Vote for MIM
I for one feel that the MIM parts are a huge step up, I only own one gun with MIM parts, but they do fit much better than my older models. And, Yes I have had the lock works out of all my other guns at one time or another. One of the biggest things I noticed is how much smoother...
Crud Removal
I shot my 617 all the time and this just what happens, how I get it off of mine is witha little scrapper I made using an aluminum exato knife handle and a short brass shim in the blade slot (cut out of a old cassing).
A poster sugested using a dental pick, I would stay away...
Not hard to find
Not sure why you have an issue finding a 158 gr bullet, they have a number of offerings sitting on the shelves everyday at Cabelas or the Bass Pro Shops. A qucik internet search should have a nubmer of mail order places for you to purchase from.
I agree with what was said above regarding the crimp beign the issue, and with what Nightshade said, get a Lee Factory Crimp Die, which resizes the case after you crimp and you shouldn't have this issue at all.