Have an M nib 580 and its killing me.
Same kind of issue, writes fine..then you notice a gradual drying...the ink not as dark coming onto the paper etc...then it gets dry enough that you feel it affect the smoothness...then a half downstroke skip or hard start.
Sent it back to the Shop I bought it from in the UK (with my Diamine ink still in it)....they telling me "we have been using it on and off for days with no issue", we can send it to TWSBI for you but the pen seems fine"....so got it back and its at it again.
Ordered a new nib and asked the place to set it up as wet and smooth it......
In the meantime pulled old nib , rinsed out whole pen with mix of warm water and drop of washing up liquid (dish soap) - rinsed thoroughly. Filled up with a different Diamine blue ink. Ran a razor blade down the feed, brushed with an old toothbrush and rinsed and put back together. Wow...I seem to have fixed it......for a while.....then it starts again to writer lighter etc
New nib unit arrived....emptied and refilled pen....seems good.....for a while.....then gradually drier etc etc
Razor blade down new feed....a bit better for a while......then back to starting to get drier etc etc
This pen is killing me...I've a old Cross Townsend that is lovely and smooth and wet, same for a M nib VP, same an old Sheaffer Cartridge pen.....but this 580 starts out well then gradually after writing maybe 2 or more A4 pages it starts to get dry......then an odd skip or hard start
Sending back to TWSBI (from europe) will cost me yet more money, after already posting it back to UK shop...and buying a new nib unit....but I also "razored" the feed (so probably invalidated warranty)........is there any saving this pen ??? I'm thinking of sending to a Nibmiester like JS in the UK but is the bloody pen worth it ?????? but I really like the colour and size it......its the RBT model .
No wonder some people have been put off fountain pens pens for life if their only experience or FP's behaved like this !!