Well, they have a schematic and block diagram to help them find errors, but I have to say that Kudelski was very picky about what they used to build Nagra.
We are not talking about an Uher Report or a Revox, Telefunken Studer...
Normally I always change electrolytic capacitors, but Nagra has electrolytic capacitors that were really only found in military applications.
I know also no Nagra which quit the service because of broken Elkos. If you have a component tester, unsolder the suspected electrolytic capacitors, test them and replace them if they are defective, but I don't think that the electrolytic capacitors are the problem.
My Nagra shows a similar behavior as theirs, probably not quite so extreme but still. This is because a tape machine must be calibrated to a specific tape, then the level is correct again. The procedure is explained in the German manual, although not as detailed as one would need it.
Try (if you have) different tapes (sometimes one tape is no longer good).
I have found that mine has problems with Agfa tape, BASF seems to suit her (especially DP26), Pyral/RMG/RTM LPR35 she also likes to.
Try to find out where your Nagra came from, most of the time you can find out what kind of tape the previous owner calibrated the machine to.
Mine came from Radio Bremen or NDR, which means the machine was most likely tuned to BASF DP26.