Crank no start

Turns over won't start. Replaced fuel pump relay. Did not help. This has been on ongoing problem for one and half years. Been to three repair shops when it was running but doesn't show on diagnostic machine. Has started twice with starter fluid. Sometimes it will start fine for months. During a rainy month of downpours it started every time. One thing seems to be when it is parked with the front down on an incline. Another I let it idle for fifteen minutes then drove it one mile turned it off next day wouldn't start. I normally keep fuel tank half to full. I have a new interstate battery that works well.