My 02 dakota has 267,000 K Frequent oil changes. Recently changed 4 days before this.........

The oil pressure has been dropping to zero then back up again and so on.
I faintly heard lifters ticking, I immediatley shut it off and have not driven it since.
Any help would be awesome.
I am getting a mechanic to take a look @ it tomorrow Aug 11 08
Where should I start?