My misfire indication became predictable--whenever the outside temps dropped quickly overnight I could count on the rough running and CEL coming on. Same things over and over; re-start after engine was warmed and it was gone until next temp drops etc. Even on the warmest of summer days a drop of 30* (not uncommon here) would trigger it too-----winter is was a bit sporadic but the re-start would "cure" it.

Start the engine and direct short bursts of something like MAF cleaner spray at the various vacuum connections and observe if your shudder is affected. This process "plugs" a vacuum leak long enough to effect a difference---its been used a long long time for such things. If there is a bad connection or deteriorated hose/line the effect will be repeatable which helps isolate leaks etc. Its worth a try.

Once triggered then CEL will go out by itself only after 10 times of restarting the van after whatever triggered it originally has been corrected, whether by you or by itself otherwise a scanner can be used to clear the code. Its unusual it will go out in just a few days unless you start/restart that often?