the "solid" type of capacitors don't typically fail. (ceramic, mica, etc) even after 40 years they are still good. the solid tantalum ones popular in the 90's don't usually drift out of spec, they just fail.
the electrolytic ones quickly go out of spec. the electrolytic capacitors have a usable lifetime measured in HOURS not years.
I think at this time if it still has it's original capacitors from factory, it's time to swap them out. check out "bob roberts" page for links to kits you can buy that have instructions and all the caps you need.