i don't know about marvel mystery specifically, but some cleaners recommend a certain schedule to use them- like stp's complete fuel system cleaner says not more often than every 4000 miles- but if marvel doesn't say- i'd keep using it. i use chevron techron about every 2-3,000. once a year, usually spring, i use SeaFoam in a tank as i head out on a highway trip.

cleaner or not, there would have to be 'junk' in your tank to be broken loose by the cleaner- that would still worry me a bit

 

See if you can get your hands on a known-good IACV and switch it into your car temporarily. If it idles fine then you know you need to replace your IACV. Otherwise you can buy a new one and if it doesn't fix the problem then you could try to sell it or something. But I would try to find a known-good one first.

Also you may have something as simple as the vacuum hoses running to and from the IACV being messed up going on.

 

the vacuum hoses on my iac (when I cleaned it) both had coolant in them, one of the hoses runs to what i believe to be the fast idle thermo, the other i dunno
1. is there suposed to be coolant there(i think yes but i am uneducated)
2. Should a manual tran engine have the FIT?

 

Yes the IACV has coolant lines plumbed to it. All cars have a fast idle thermo valve regardless of AT or MT. The thing that only AT cars have is a starting air valve.

Like I said man it's probably a bad IACV.