Windstar bucks, need help identifying problem

Just a few days back my 99 windstar started bucking while driving around and especially on the highway. The rpms do not increase or decrease and the van appears to run fine while the bucking is happening. We can get it to quit the bucking if accelerate. It does not do it very often.

Any ideas on what this could be?

Thanks

edited to add: the check engine light is on. Last week took it to autozone and they said it shows lean on 1 and 2. I dont know if this has anything to add but I felt I should add it. I added a fuel injector cleaner as well.

Were the codes 171 & 174?
Do a search on this...and you will find that your windstar is part of a TSB.
There is a LOT of information posted here about this TSB. If you are handy....you can buy the parts and do it yourself.
This TSB includes new bolts for the upper intake manifold, new upper intake manifold gaskets, and a new valve cover.
The new bolts hold down the upper intake manifold better.....preventing vacuum leaks...which can cause the issues that you are having. The new valve cover includes an improved baffel by the PCV valve, to prevent excess oil from being pulled into the PCV valve...and thus into the intake manifold.
The TSB also includes a reprogram of the computer....you would have to go to a FORD dealer to have that done.

My van did the same bucking and jerking. Turns out the vacuum line going from the upper manifold to the fuel pressure regulator came loose. You probably still need to do the whole TSB as I eventually had to do.