I'm new to this list, so let's see if this works:

I've got an 87 2.1L vanagon which i've been working on.  I rebuilt the
engine, and replaced most the cooling system. (the old owner ran the
coolant dry and a valve overheated/jammed/bent pushrod/bad stuff.  Anyway,
once my bentley manual and I finished with it, it runs pretty good, BUT
when it gets warmed up, it seems to have low power issues.  That is, i have
to shift sooner, and the accelerator seems to do nothing at low RPM.  One
more thing:  When, and only when it's warm, the high rpm oil underpressure
warning light and buzzer go on around 2000 rpm.  Above that it's fine,
below that it's fine.  Has anyone seen issues like this before?  It's so
confusing to me, because I replaced the oil pump, all the barings, the
rings, and the oil doesn't leak.  Any suggestions?  Thanks

Marshall Taylor