Your Ace 250 is a $200 detector. It does a lot for that price, but it isnt more than that. I have one and find its tendency to false notable. One of the problems is it is quite sensitive. I turn it down a bit, but it does like to announce things, sometimes if they are iffy. One remedy for this is to ensure it is a good signal by scanning it from various directions for repeatablity. Junk signals will be jumpy and inconsistent. Like all Garretts, it takes some getting used to. Once you have though, youll find it capable.
As for bottle caps, well, I dont know too many detectors that dont take a liking to some or another of them. Steel washers, too. Especially if you hunt in the lowered DISC modes. Large iron items will cause falsing, too, but it is usually noted by the fact that the cursor is jumping around, normally into the iron range and then back again. Also, the audio will blast and chop, but wont smoothly signal.
Try running in ZERO DISC for awhile and you'll see what a problem iron masking/overload REALLY is. The stuff is everywhere.
You tried the reset and you have strapped the cable to the shaft. Both good things.
The cable strapping is, oddly enough, very important. It does make a difference as you noted and I do it now to every detector. However, ensure it isnt so tight that you remove any strain relief. If you do THAT, you will soon have other problems with your coil.
The GB is preset and as such is made to function in average soil mineralization. If you live out west or in some of the cinder zones in the NE, etc., you may experience some trouble. There are those intrepid folks now who are adding external GB pots to their ACE, but I wouldnt call that a science yet, and Garrett schematics are notably scarce.
I personally wouldnt buy from Kellyco, but lots of folks do. I know of some who got defect models as new and had to go through the wringer to get it right. Whats done is done in your case. Mary Penson at Garrett can help a lot and is a sweetie. She's due to retire soon enough, so make sure you dont delay too long if you need help!
Bottom line: modern discrimination is both a blessing and a curse. It solves some problems and creates others in trying to to 'do it all'. I suggest you hunt in your old "fashioned ways", ie, in Jewelry and Relic modes for 30 hours or so - and in ZERO as much as you can tolerate. Avoid the Coins mode for the most part. In the end, youll learn more and it'll pay off in the long run..