Fixed. It was the igniter assembly. The pressure switch initially tested fine with the manometer, but after installing the igniter something went wrong. So, I then tested the switch to make certain that the draft inducer was working properly--and it was. When I troubleshooted the control center everything ran fine--the test sequence indicated that the problem wasn't with the control center. To make a long story short, the problem was entirely my fault…I left my husband alone in the basement with the furnace. He was inadvertently switching the hoses to the pressure switch. This is why sometimes the furnace would run fine (during test mode) and then it would stop working. The last time he did this I asked how it was that he could not read the numbers on the meter, but had no problem reading the positive sign on the switch. We then realized what had happened. I feel silly posting this, but perhaps someone will learn from this mistake. Thanks to everyone for all the help.