Intermittently running rough, stumbling

[TrybalRage]

Hey everyone. Struggling and getting a little frustrated and hoping for some advice. I think I might be on the right track but not sure.

I have an '85 C10 with the 305 (glovebox code 1LE9)/TH350C and I have had some weird rough running issues that come and go. A week or so ago, I changed the oil and adjusted the timing a little. It had been pinging very lightly under load for a while, and when I checked the timing it was about 15° BTDC. I adjusted it slightly to about 8°, tightened down the distributor, and went about my day.

A few days later I was driving it to/from work, and on the way home it seemed like I had no power, having to go really heavy on the pedal just to get it to move... it seemed to come and go.

When I went to check the timing, it was WAY off... like 6° ATDC-ish... off the marker. I thought maybe the dizzy had rotated or something.. maybe I hadn't tightened it down enough. So I adjusted it again and it immediately started running better. I went around and tinkered with a few other things... and suddenly the RPMs jumped (a lot) and I quickly checked the timing again, it was now up at maybe 20° BTDC. WTF?

I rotated the dizzy back to where it was, and got the timing back to around 8° again. Clamped everything down and wrote it off as a wierd glitch.

Well the engine still had the intermittent rough running. When I got home the one day, fighting in traffic and having to put it in neutral at every stop to keep it from stalling, when I started trying to find the problem I noticed that the exhaust sounded like it was choking or muffled. I checked the valve/flap on the exhaust manifold and it was not being opened by the vacuum. Seemed like either (A) there wasn't enough or (B) the temperature switch on the thermostat housing was malfunctioning (it's less than a year old). I bypassed the switch and directly connected some of the vacuum lines and the valve opened and immediately it started running better.

I started looking around at other stuff seeing if I could just bypass some of the emissions stuff, and while I was under the hood (with it running in park) it would keep switching back and forth... running rough, idle erratic, then smooth out and run fine again. I got my timing light back out and saw the same type of changes as before. Thinking that the vacuum switch perhaps was affecting the vacuum advance in an adverse way, I bypassed it completely (4 lines into the temp switch). Seemed to help, took it for a spin and no issues.

This morning on my way to work, it started up fine, and then acted up again. Sluggish, feeling like the engine was choking when I step on the gas. Even putting it in neutral and goosing the pedal it feels like its missing on a couple cylinders. I limped it to the office and when I was within about a mile all of a sudden it started running OK again.

So here I am. Trying to figure out what in the world would cause these issues to come and go. Been researching online and I'm starting to think that the ESC could be an issue. From what I see my engine code indicates that I would have that, and since it is there to change the timing when knock is sensed, could it be possibly faulting and jacking with the timing at random? Or am I looking in the wrong area? Anyone else experience anything like this?

Sorry for the long post, just wanted to try and include as much information as possible.