I had pretty much the same issues when I bought my second hand Infinium, but have managed to resolve all issues with minimum cost. It shouldn't be EMI, as the Infinium is quite stable in that respect, I can operate only a 100m away from mobile phone towers, and run it in my powered metal workshop with no issues.
Could be a few things, either an issue with the coil cable plug on the unit itself, mine came loose on the inside and cause continual hi/lo falsing no matter what coil is used, that was pretty cheap to fix at the Gold Mining Centre here in Adelaide.
One thing to consider is that whether the pins on the coil plugs are actually entering the small holes on the unit plug properly, you have to be very careful to make sure they go in those holes, and not into the surround foam of the plug, otherwise it will cause falsing. If you check the plug on the unit, you can sometimes see holes in the foam where the pin has missed the proper metal in hole, it is very easy to do. This may also be caused if the pins on the plug are a bit bent, or if you are just in a rush to connect everything up. Be warned though, don't bend the pins too much as they break very easily, requiring a completely new plug to be fitted. I like to physically see the pins enter the proper holes, then partly tighten the round screw on bezel, then check again to ensure that the plug is pushed all the way home, then a final tighten of the bezel.
Another issues with these detectors is if it is an old unit that has not been upgraded to run mono coils, the previous owner may have blown the front end electronics on the detector (and not told you about owning a mono). All Infiniums sold through Australian dealerships in the past should have had this upgrade, but if bought from abroad, may not have. Don't fret though, it's not very expensive to upgrade those eletronics (approx $50). Upgraded units from Australia should have a yellow or green sticker on the inside of the battery compartment.
If unsure of anything, or to confirm whether it is indeed an Australian unit, or if it requires upgrades/repairs, contact Phil at the Gold Mining Centre here in Adelaide on 08 8936 7455. He is an electronics technician, and used to be the Australia wide distrubutor for Garrett. He is extremely well versed when it comes to the Infinium, and doesn't charge the earth to look at or fix things, so it may pay to pick his brains.
Don't despair, it could be something that's easily fixed.