Honda Passport intermittantly won't start

I have a 94 Honda Passport, 6 cyl. I’ve had it for almost 5 years and, since the beginning, its done the same thing. It runs fine most of the time. But about 3 or 4 times a year, when I try to start it, it will make a very hard “boom” (my husband says its a backfire but it seems worse than that to me) and the whole car shudders and does not start. The first time it happened I thought the engine had blown. After it makes the boom, it won’t start. It turns over and if you pump the gas it will almost start but if you release the ignition it stalls immediately. Occasionally if you try 5 or 6 times it will start but it sputters for a minute or so and then stalls. When it does that, a few more tries will usually result in it running. Within a minute or two of it running, it runs like nothing ever happened. Weather doesn’t seem to make a difference. If you don’t get it to do the sputtering start, its not going to start unless you leave it alone for 15-30 minutes. Then it starts like nothing was wrong.



I’ve asked mechanics about it but, since it never happens when a mechanic is around, they have no idea what the problem is.



Any suggestions?