De filter close to 20 years old

Do you have an MPV with RECIRCULATE on it?

You can replace your filter with another FNS DE if you are happy with it.
 
It could be your grids are full of grime. Have you replaced the grids?
Older filters had fiberglass on inside of the housing which can deteriorate over time.
I did a replacement with another DE filter. Although the pipes lined up from the MPV, there was a slight height difference. I had to put the new DE filter on stepping tiles about 1 inch high. So make your measurements and be sure it all fits.

Moving to a cartridge filter will require you to move the plumbing.