Mark Hama is THE Yashicamat person in the US as far as I know. He's also local to me. I've corresponded with him but ended up getting a camera that didn't need service. He quoted me $155 for a CLA on a 124G last year. He also sells CLAed Yashicamats on eBay with a six month warranty for, IIRC, about $450. Pricey to be sure for a 124G but you'll have a camera that's right and needs no work, with a warranty. (He doesn't have any listed now but does from time to time.)

I don't know if this is common. I haven't noticed it with mine nor have I had the shutter timed, but I also don't use the highest speeds that often. My feeling would be to either spend the same (or a bit less, I paid $158 for my 124 non-G which I prefer anyway) for one that isn't that far off or spend a bit more for one with an effective CLA.

Here's Mark's web site in case you are interested in asking about servicing the shutter:

http://markhama.home.comcast.net/~markhama/