Interesting - didn't know that about slowing down. The attraction is pretty strong - the instructions say any tank over 1/2" should not use the spacer. I believe my glass is 15MM, which is right around 5/8" (tank was built in Europe where dimensions are metric).
It failed one more time at just about 50%, so I turned it down to under 50% (its all a guess since there is just the little dial). I do wish there were markers on the drivers, like 30%, 40%, etc - hopefully on the newer models.
The Vortechs aren't that old, they should be the new code according to a friend, Philwd. Hopefully the issue clears up, I'd hate for a pump to go out for a few days while I'm on the road since I travel quite a bit.
Thanks again for all the help,
David