HELP with Apple II+ booting!

This seems to be the weekend for Apple II series boot problems! My Apple II+ will boot a diskette called the "Zardax Utilities" but it won't boot anything else. When I boot with the Zardax Utilities diskette, I get "APPLE II" on the screen, some disk activity, then the Zardax menu comes up. When I put in a different bootable diskette, I get "APPLE II" on the screen, and then after a moment's pause, a bunch of garbage characters are added. This happens with various bootable diskettes, including some brand-new shrinkwrapped Atarisoft games (whose docs say they are bootable). With the garbage on the screen, if I hit Shift/Break or Ctrl/Break or whatever, I can get to the ] BASIC prompt. I've switched out the Drive 1 and the diskette adapter card. Any idea what's wrong? thanks Kai