First question - how many miles are on the vehicle? How many miles since the last engine oil and filter change?

As long as the engine oil level is okay, then the next steps would be to: check/measure the engine oil pressure, and check for any engine oil leaks.

It may be the engine has enough wear due to mileage - that the oil pressure is low at lower engine speeds. Your options are twofold if this is the case; repair / rebuild the existing engine, or replace the engine altogether.