Possibly a little bit of sludge is blocking the oil hole to a lifter. At idle it may be able to get enough oil to stay pumped up, but running down the road it may not have enough oil flow to the lifter to keep up with the work it is doing. Kinda like the restricted water lines in my house from 60 years of use. You can flush the toilet, or wash dishes, or use the washing machine, but not any two at the same time.
May have to clean the lifters or any sludge built up in the engine. Someone else should be able to shed more light on your issues.
