Sounds to me like classic symptoms of a dirty drivetrain. If your chain is getting slack on top when you stop pedalling or backpedal, then the problem is in your freehub (or freewheel, whichever you have), a dirty freehub will cause all the problems you've listed, even on a brand new bike.
When dirt and grit mixed with excess chain oil get gummed up in the bearings of your freehub, it looses a lot of it's freedom to move. The easiest way to confirm this is to take your wheel off, and just try to give the cassette a flick with your fingers, if it doesn't spin freely, then it is certainly gummed up and the easiest thing to do is get the freehub replaced (freehubs aren't easy to clean unless you have a freehub buddy). You can clean a freehub without a freehub buddy, but you'll have to take it appart to do a good job of it, and there are lots of tiny bearings (~42) in a freehub that are easy to loose, so if you're not mechanically inclined, I wouldn't recommend trying it. If you want to try, you'll need a freehub tool to get the freehub off the hub, and another to open the freehub (their different depending on brand of freehub), I also recommend getting a pair of snap-ring pliers and a magnet tray (pliers for snap-rings obviously, and magnet try to prevent dozens of tiny ball-bearings from going everywhere).
How to Prevent Sticky Freehubs
Proper oiling and regularly cleaning your drivetrain is the best way to avoid this problem. Over-oiling is the biggest culprit for sticky freehubs, don't just keep applying more oil to your chain, always wipe off the excess, and clean you chain regularly to remove dirt and grime, simply oiling isn't enough.
Riding your bike as often as you do, you should probably be getting your drivetrain replaced at least once a year (Chain and Cassette) as wear and tear will also give you problems.