1990 Pontiac Bonneville 3.8 wont turn over right away

1990 Pontiac Bonneville 3.8l mileage: 98,000. My question is similar to the person with the 88 Pontiac Grand Prix. After I drive the car to work, twenty miles or so, I turn the engine off leave it until lunch, about five hours. The engine does crank and all lights and electrical work, but it just wont turn over right away. Whenever I take it to a mechanic, the car does fine! When I try to start it, it cranks, a couple of times it tries to turn over. Eventually, it will start after forty five seconds or more of cranking. I had the fuel pump and fuel pump sensor checked and the throttle body cleaned (mechanics suggestions). It gets good spark. I had someone tell me there is a crankshaft-positioning sensor that was a problem on 1990 Grand Prix. Could this be the problem?