2000 Subaru Outback - Stuttering uphill / upon acceleration?

Do yourself a favor: If the exhaust isn't welded on, remove it and check the pipes and catalytic for blocks. If it's not blocked, that's definitely not it. First thing the shop should have checked too if you ask me! Now, check the condition of the O2 sensor. If you're not sure what it's supposed to look like, go to the parts store and ask them as they will know for sure. As for the valves, well I'd just dump in about a quarter bottle of Star-Tron before I do anything else because of the ethanol that's in the gasoline these days. And as for the automatic Transmission problem: Either get it to a shop and have them flush the and the coolant lines before they re-build it, or swap it out for a manual as the automatics are VERY expensive to work on once they've begun to give you issues.