There’s not a lot of information here to go on and Scottie and Shutterbug have already mentioned the obvious. Maybe you can shed a little more light on the problem. You say the clock rings for 5 days and that it runs strong, but how is it running at say, 4 ½ days? That is, does the pendulum start to swing less and less over the five day period so that it is noticeably less on the 4th day and finally quits on the 5th day?
If there’s nothing actually binding the movement, like the spring expanding against an arbor, then I would suspect a power problem. That is, either the movement is requiring too much power, or the spring is not able to provide enough power after 5 days running. Either way, I believe you need to take the movement apart, completely apart, including removing the springs. Check for rough and/or bent pivots, and excessively worn pivot holes. As previously suggested, a damaged, poorly fitted, or dirty pivot will cause a lot of extra friction.
Check the springs carefully. Are they rusted, rough, dry, or coated with dried up oil? There’s no absolute rule, but when you lay out the springs from a clock like this, if they don’t expand to about 6 inches or so, the spring may be “tired” or “set” and no longer able to sustain power for a full 8 days. You say the springs do not appear to have been shortened. Did you measure them or come to this conclusion because the end looks “factory made”? It’s not uncommon to find old clocks that have had incorrect springs installed. Did you lubricate the springs, and if so, with what?
Some pictures might help, along with dimensions of the springs – thickness, width and length (don’t try to uncoil the last few turns). I believe that if you take the movement completely apart and reclean everything, and carefully inspect the pivots and pivot holes, and the spring, you will easily identify the problem.