1995 Lincoln Town Car hard starting

Within the last year, at least once a week I have a hard time starting the car. It cranks and cranks but but it will not turn over. Sometimes, I have to wait 5 or 10 minutes and then it will start. Yet, sometimes it starts after the 3rd or 4th crank without waiting. There is no ryme or reason or logic or consistency with the problem. I took it to my local mechanic and he checked the starting system: battery, starter and alternator. All readings met or exceeded the minimum requirements. He found no problem and suggested that I leave it for a week. The next time I had the problem, I took it to my local Ford dealer since I thought they might know of a recurrent problem with this model and year. They ckecked the starting system again and found no problem. They asked to keep it for a few days. They then told me that it was the fuel pump. I gave the Ok to change it. It then worked great for about two weeks. I went on vacation two weeks ago and the problem returned. I have owned the car since new and take good care of it and has absolutely no rust since I only drive it from April through October since it does not handle well in the Chicago snow because it is rear wheel drive. Most miles are highway. 99.9 percent of the time it starts and runs great. I could live with the problem but sometimes my work takes me or my wife through some very rough areas of the City. Can you help?