I've never experienced atomizers that stop working after a few hours, apart from the odd bad one which never worked right from the outset. Are they failing completely (i.e. dead, stone cold, not functioning at all) or is it just decreasing in performance after a few hours? For the former, there's not much you can do apart from replace the atomizer. If it's decreasing in performance, the obvious issues to check are that the battery is charged and that there is liquid in the cartridge. If the
batteries you are using are a year old, it's likely they are going to have lost much of their capacity, and they might be the issue. Try swapping for a fully charged battery as soon as the vapour tails off, and use a fresh cartridge. The cartridges in this model are very small and do not last very long.
With the KR808D-1, there are a few options as far as cartomizers go, which might be worth exploring. The original Kanger KR8 cartomizers seem to have quite a strong
throat hit, but without producing a great deal of vapour (in my experience). There are others available. For example the "
Premium Coolcart" range from Vapor4Life is said to run a bit cooler than others. Alternatively use blank premium cartridges (from say Vapor4Life or Litecig USA) and fill with e-liquid of your choice. The liquid itself makes a big difference to how harsh or otherwise the vapour is.
If that's still not suitable, then perhaps an old favourite from the same era as the DSE103 might be worth looking at - the DSE901. It's commonly available, and will be easier to find spare parts for than the DSE103. I've never used a DSE103, but in my opinion, the 901 is less harsh than many others, and might be what he's looking for. It's certainly easier to refill the cartridges with these than the DSE103's, and refilling the carts yourself with e-liquid is much more economical, and gives the opportunity to use many more e-liquid flavours and strengths, than using pre-filled cartridges.