do you have a intake manifold with the vacuum port on it? might be time to hook up a vacuum gauge while its limping - that can tell you HOW the valves are closing among a bunch of other things.
https://www.google.com/imgres?imgurl...HaCJMQ9QEIKjAA
stupid question but are you sure a critter didnt make a home in your muffler while it sat for a month? restricted exhaust can cause all sorts of issues (vacuum gauge will show that as well)