Chrysler Repair: Dodge B300 vibration, cyclical manner, drive shaft

Question
Hello,

I have a 1978 Dodge B300 Van (318, automatic). Recently, there is a strong vibration but I am unable to determine what is causing it. I do not think it is the drive shaft because I can feel it even when the car is immobile. The vibration is "constant" but "cyclical". The cycle goes like this:  
1)no vibration for 3-5 seconds,  
2)vibrations starts and gets stronger and stronger during about 3-5 seconds  
3)vibration gradually fades away, which takes another 3-5 seconds  
4)cycle restarts

Any information would be a great help!
Thanks!

Answer
Hi Daniel,
I wonder if it due to a cylinder beginning to miss and dropping out in a cyclical manner (but I can't explain why the cycling) OR is the engine proper running evenly but the engine suspension system compromised in such a manner as to have a resonance sort of phenomenon which is cyclical? If the engine seems to give you steady power output then my thought would be to check the condition of the rubber/metal inerface engine mounts to see if one or more of them are broken, or stretched to the breaking point, or loose. Because I haven't experienced such a syndrome and I haven't had much experience with vans, much less v-8 rear drive vans, I am afraid that is about the only suggestion I can offer you. One way to assess the mounts is to observe how much movement the engine shows when you rev it or when you shift in and out of drive/reverse to the neutral position. It should not rock very much if the mounts are good.
I would be curious to know if the mounts turn out to be the answer, and if not what is?
Roland