2003 GT does not have turbo (unfortunately), so no turbo - no vacuum.
Subaru Canada suspected carbon build-up on 2-nd cylinder valve, and recommended 2-3 tanks of Sunoco 94 gas. They have additive to clean valves. Let's see how it will go. If after 2-3 tanks of 94 I will have the same problem - valves need to be replaced (no cost for me, full entended warranty)