Q - What can I do about my Unrepeatable slides that are overexposed, underexposed or just plain blah?
A - You can improve them through copying. Underexposed originals can be corrected by overexposure; overexposed frames by underexposure. Flat-lighted scenes can be pepped up by the increased contast gained through copying, and dull colors brightened through the use of filters. you can't cahange all your dogs into dillies, but you sure can save most of them.
There are many ways to make copy slides or duplicates. A very simple system works for me and should work for you. For a copy lens I use a micro-Nikkor 55mm with a 1:1 close-up ring. Any lens with a flat field coverage and a 1:1 shooting ratio will do.
I use my light table as a copy stand. This way I can see the transparency for focusing and judge the required exposure and filtration. To actually shoot the copy, I place a small strobe inside the light box and point it up to flash through the transparency.
I use Kodachrome 64 or Ekatachrome 64 for copies. I prefer the Ektachrome because it is less contrasty. You can use any fine-grain outdoor film like Fuji, Agfa or others just as well.
My exposures run between f-16 and f-32 at X strobe sync speed. You can use this as a start but make a test with your own setup and equipment by shooting a series of copies of one of your good transparencies with a half-stop difference between exposures.
Filtration is a matter of individually eye-balling each transparency to see what correction is needed. You can do this by stacking color-correction filters under the slide and judging the effect. Color-correction filters can be bought in sets-sets of different colors and graduated densities. You don't have to buy the most expensive ones for this purpose. Yellow or magenta, or a combination of these two, is the filtration that most transparencies require, and you need to include a 2B ultravoilet absorbing filter in the filter pack under the slide.
My system will give you good dupe copies for viewing and slide projection. If you want to go on to finer things, try the Eastman 5071 duping film. This is a stickier medium but it can give you repro-grade dupes.
Kodak won't tell you the ASA, but it's in the ballpark between ASA 12 and 24. You'll have to make your own tests as each batch varies.
The light source for 5071 should be 3200 Kelvin and the exposure range is from f-5.6 to f-11 at a half-second. It also requires a 2B filter and a high correction of yellow and magenta.
Slide copying can be simple or complicated, depending on your needs. Either way it can be a lifesaver for your dead pictures.