28 wt is an awfully heavy thread---it will be difficult to equal the tension (top to bottom) with such a huge difference- and your fraying thread may be because the needle eye is not large enough to really handle such a heavy thread.
if you get the top & bottom threads closer in weight (main reason it's recommended to use the same thread top & bottom) you will have better luck getting the tension right.
i clean between my tension disks when ever they need it- and have never had that cause a problem later-unless i turned the tension dial while doing it.
i would try a 40wt thread top & bottom and work on getting the tension back to correct---then when you have it equal go ahead and go back to experimenting with different thread- make notes of problems, with different combinations- but remember you also have to change needles with different weights/types of threads.