The tach switch "contacts" can get dirty, leading to a wrong reading. I'm thinking this is "probably" not the RPM issue, but it's a freebie to check it out... just to be sure.
If the gas is bad enough, it is unlikely that Startron (which I DO like - I use it myself regularly) will "rejuvenate" the gas. Smell it and dump into a glass jar. Don't guess - do it.
In newer EFI systems (newer computers), the IAC will sound the warning horn. I'm not sure if it does so with your MEFI-3. Some sensors will, I'm just not sure with this one. See what happens (you won't do any damage) if you disconnect it while the engine is idling (horn and/or dieing).
Could be a loose throttle cable (at engine) causing this, too.
In the end... I think you're probably best getting it checked out by a Merc technician with a CDS scanner.
If the gas is bad enough, it is unlikely that Startron (which I DO like - I use it myself regularly) will "rejuvenate" the gas. Smell it and dump into a glass jar. Don't guess - do it.
In newer EFI systems (newer computers), the IAC will sound the warning horn. I'm not sure if it does so with your MEFI-3. Some sensors will, I'm just not sure with this one. See what happens (you won't do any damage) if you disconnect it while the engine is idling (horn and/or dieing).
Could be a loose throttle cable (at engine) causing this, too.
In the end... I think you're probably best getting it checked out by a Merc technician with a CDS scanner.