Hi Tycho,

check both capacity of oil pumps (l/tr), could be different. When running conditions change, if nominal oil flow is to low, oil pressure decrease suddenly.
I have monitored the oil flow on a compressor for such a trouble; when pressure ratio increased, oil pressure dropped in a few seconds. oil pump KF3 had been replaced but wrong gear length used so nominal oil flow was reduced.
your oil pump can also be worn.