1999 Ford Expedition Oil Pressure Drops

I recently had some issue with my Expedition and spend about two weeks working on the porblem. The initial issue was that the Expedition stalled out and would not restart. I determined that there were a few issues that had to be resolved. 1) The engine seems to be partially siezed 2) The battery cells were not strong enough to turn over the engine. 3) The alternator needed to be replaced 4) The Expedition was blowing a lot of white smoke after the engine finally turned over.

What I did to the Expedition. I replace the battery, replaced the alternator, flushed the radiator with a block seal and refilled with all new anti-freeze, changed the oil (to a pure synthetic) and filter.

After all was said and done, I drove the vehicle for two days (over the weekend and around home) with no issues. I then drove to the office (about 35 miles) and the oil pressure dropped and the light came on. I shut down the Expedition on the shoulder of the road and the pressures returned to normal. I have done this now for two days. After about 20 miles of driving, the pressures will drop and the light comes on. I shut off the engine for a moment or two and then restart the Expedition. The indications are all normal after that for some time. What should I start with and how can I be certian that there isn't a bigger issue?