3/8" beer line is too big for home use. You need to switch to 5/16" to get the line resistance you need. You have CO2 coming out of solution in the beer line plus you are pushing the beer too fast due to the lack of restriction.
The two basic problems are that you don't have a balanced system and that your "shake and bake" carbonation process is inconsistent. Here is a great reference for figuring out how to balance your system and to troubleshoot problems.k http://kegman.net/balance.html
Wayne
Bugeater Brewing Company


