just a quick thought - as you know when your sparring hyour wasting a lot more energy then normal training. Is it possible you were not rehydrating enough. The number 1 cause of headaches for me was always under estimating how much I needed to rehydrate (and pre-hyrdate too). I know for me drinking more water before and during the round breaks helped a lot.
Personally, bridges have had a greater impact on my overall neck strength and size than weights with a neck harness. I use the moving neck bridge in this video: [ame]http://www.youtube.com/watch?v=u6zlaIl0yh0[/ame] I do the same thing on the front, although on my front I do it from my knees. If you try to do a standing moving neck bridge from your front you can potentially do some serious damage. In the past 3 or 4 months since I made the switch from head harnesses I've added about an inch to my neck, which is alot aesthetically - On a related point, I've also become more difficult to choke. I do them in 2 sets of 45 seconds (about 2 or 3 times a week) trying to get in as many reps as I can. Always be careful though and listen to your body, because it's your neck and you do not want to **** it up.
neck raises i think there called.. lay flat on your back and raise your neck straight up, minimum 60 reps, then lifting your neck right side(and left) to center, 60 reps minimum, or turn your neck all the way over in one motion like your turning your head from side to side looking to each side. 60 reps