Sticking points in the pedal stroke

Either the rear wheel or the crank bearings are binding at some point in their revolution, or (or possibly there's something going on with the chain). The first thing to do is track down where the problem is.

Check first for obvious stuff, like is the rear wheel out of true and rubbing the brake blocks?

The next easiest thing to do first is take the rear wheel out, remove the quick release skewer and check the bearings. Turn the axle with you fingers, does the axle have a tight spot, do you feel excessive resistance or 'notchiness'? Push and pull the axle about to check for play, if you can feel the axle moving more than a tiny bit there is excessive play.

If the wheel bearings are is OK, check the crank. Most front derailleur cages have a small screw that allow them to be opened and the chain removed. Get the chain out of the cage and off the chainrings (it's awkward but possible). Spin the cranks by hand, again checking for a tight spot, excessive resistance, 'notchiness' and play.

The wheel is the problem fixing it depends on the type of bearings. Some hubs have 'serviceable' bearings that can be disassembled, some have pressed in cartridge bearings. If the crank is the issue, a new bottom bracket is required.

I graciously disagree with the troublshooting approach already mentioned. Not that there is anything wrong with checking the things mentioned, but because the OP described the chain going slack on the top when pedaling backwards, there are other things that should be checked first.

How worn is your chain? probably a lot after 2500km. I have found that a new chain can do wonders even when we don't think it should matter. There are two ways (probably more, but two that I can think of) to measure a chain with out a tool. Method one...use a yard stick. Pin to pin is supposed to be half an inch. Line up a pin with the one inch marker on your yardstick. If, after 20 or 30 inches the pins don't line up any more then you need a new chain. Method 2, use a new chain. Buy a new chain...if you are riding 2500km, you'll need one anyway. Count the links on the old chain and break the new one at the same point. Now hold them next to each other. If you notice more than about half an inch of difference, put the new one on.

If that does not solve your problem, I would take the back wheel out, and spin the cassette backwards. Does it spin OK? If not, that is worthy of a whole new thread. If it spins OK, next check your jockey wheels. Get the chain away from each of them, and make sure they spin freely.

WHile you have the wheel off, it is worth pulling the chain off the crank, and checking to see that it (the crank) spins freely. Because of the chain slack at the top, I think the problem is not here, but it is worth checking.

Another thing to check. With the wheel in, and chain on, pedaling backwards...does the chain come off the the jockey wheel and get wedged in the derailleur?

If you still have this problem, or you want to ask me to clarify something, by all means reach out. I love fixing bikes, even if indirect.