416C Swing Cycle Issue

I am the proud (and original) owner of a '97 Cat 416C (4ZN). This machine has performed well over it's life, and it doesn't owe me a thing. I have put almost all of it's 12,060 (as of today) hours on it. I have recently noticed that the swing cycle is exhibiting an odd symptom. Swinging from the right side (left if you are sitting in the seat facing the back) to center is MUCH slower than swinging from left to center. I can pull the lever to the stops but the response is still slow to start. It's hard to tell but I think it reaches full speed but it definetly starts slow. The symptom is less pronounced as the machine gets hotter (after it's used for a while). The left side cylinder developed a leak (through the weld that holds the threads for the head gland) but this has been repaired and that cylinder has a new piston and seals as well. While I replaced all trunion and rod eye (swing) bearings and pins (bottom of the boom too), I have not rebuilt the other cylinder recently. I have recently replaced the hydraulic pump. This problem developed about the same time as the leak, so I suspected that it may have been related (though the leak was not significant). It has taken me a while to accomplish the repairs (in my spare time). I am disapointed that the problem remains despite my efforts to tighten things up while it was apart. I have succeeded in making things feel new and tight but the sluggish swing is really spoiling my "pride in ownership" experience. Where should I be looking? Bad packing in the other swing cylinder, a problem with one of the resolvers, or maybe something else all together. I am requesting help to isolate and repair this issue. I do not have flow and pressure gauges so I may have to take the unit to Cat if that is what is required to trouble shoot this problem. I do almost all of my own repairs but I am willing to let the pros fix it if that is the best option. Thanks in advance for your replies.