2001 Volkswagen Jetta Intermittent starts

I have a 2001 VW Jetta with approx 88,000 miles. The car has been extremely reliable despite its poor track record but I am having a problem that has stumped me. Recently, when I go to start the car after it has been sitting for anywhere from about 1 hr to overnight the engine will not crank. All lights turn on. The way I can get it to start is by holding the key in the start position until is does start. Generally I will have to hold the key for 5-10 seconds before the engine turns over. Once it begins to turn over it sounds fine and starts up normally. I had the battery tested at a local Sears and the battery checks out OK. Any recommendations?