Get a magnifier and check for mites.

I didn't find any mites but I checked under the leaves and this is what they look like:

The newest leaves look fine to me. As for the yellowing leaves, it's not getting enough sunlight. It takes energy maintaining "x" number of leaves, and when it can't meet that demand, it will shed older leaves in order to conserve what little energy that it is getting.

When your plant begins shedding older leaves, I would recommend leaving the leaves alone. At least until they have turned completely yellow. The reason leaves turn yellow before they die back is because the plant is reabsorbing all of the chlorophyll and other nutrients back into the plant, to reuse in other areas; rather than having to start from scratch. It may not look all that appealing, but it's just a natural process that all plants go through. Sure, you can remove those leaves prematurely and your plant will still live, but why let that chlorophyll and nutrients all go to waste, when they can be used to help benefit your plant in the long run? Just some food for thought.

Okay, I will investigate the sunlight situation. I will move it to a location closer to the window and see what happens.

These plants love bright, shaded sunlight. In their natural habitat, they climb up the sides of trees, where they get plenty of morning and evening sunlight, but are shaded from direct overhead sunlight during the afternoon.

These have a natural tendency to spread out, looking for trees to climb up in search of brighter sunlight. I would suggest staking it as it grows. I originally used a wooden stake for mine, but it failed miserably. I eventually had to repot it using a steel rod that can support my "monster" without bending or breaking. I also used stronger plant ties made from rubber to attach it to the metal stake. The weaker plant ties that I used before never stood a chance at taming this beast, haha.