Mark I have been thinking about my own experience in engraving. There are many things that can be causing your problem and it would be hard to give you an definite answer. What I will do is tell what I did to solve this problem for me.
I first took a class from GRS. Yes this cost money but not that much. I had struggled for about 4 years trying to teach myself engraving and needless to say I was not doing to well. I learned more the first day of the class than I did in the 4 years I spent trying the self taught method.
I purchased a sharpening system. I have both the universal system by Lindsay and the GRS dual sharpening system and I use them both. You can not cut good lines until you can sharpen a graver. Many can sharpen a graver without a sharpening system, I am not one of them. If you can then great. If you want to cut a consistent lines you need to be able to sharpen consistently.
I first started with a chasing hammer that was to heavy. I was constantly breaking the tip of the graver. After taking classes for Mark Silver and Wallace Gusler I determined that I needed to use a lighter hammer and lighter taps on the graver. This is not easy for guys that are our size. It was very difficult for me to learn that I was not driving a nail and to lighten up on the force used to drive a graver.
I also was using an aluminum handle to hold the graver that was about 3/8" in diameter and about 5" long. I found that in trying to hold onto this round graver handle I was over gripping trying to control the graver. My hand would cramp in a very short time and my lines looked like $#@*. I switched to a wooden handle that was shaped more like a file handle and found that I could control the graver better and not use a death grip in trying to control the graver, thus no more hand cramping.
When I took the GRS class I used one of their air operated hand hand pieces. What a pleasure, I did not know that engraving could be that enjoyable. I now use a Lindsay Palm Graver and consider it to be one of my best investments. I know that many want to say this is not traditional but hey how many truly build in the old ways, very, very, very few. I found I could engrave all day and not be worn out and hurting. I could also produce more work and better quality work with the Lindsay. So for me now I only use the H&C if I have to. I know how and I do it occasionally but If I want to to my best work on a rifle is go to the Lindsay.
The Lindsay was a big investment but it was worth it to me. I wanted to be able to produce top quality work, as least to the best of my ability. You build professionally also and with your health issues you may want to take a look at the GRS and Lindsay systems. Both are good, not cheap, but worth every penny to me.
Good luck and I hope that his helps
Jim