Your engine is telling you it is on it's last legs.  Trust me on this.

There is no way of knowing the quality of the rebuild or how it was
maintained, but they should last at least twice this long when properly
done and maintained. The problem with a 2.1 in the '84 is the lack of an
oil cooler, which can be added with some expense and trouble, but I
think it is worth it.  If too thin oil was run (i.e., the common 10w-40)
it may not have provided adequate lubrication at speeds and temps, hence
your short engine life.

To keep this engine going as long as possible put single weight 40w HD
oil in it (fine for our NW climate in your situation with low oil
pressure) and avoid long trips.  If the light still flickers, add a can
of STP too.

Sell the engine while it still is rebuildable, these 2.1s are getting
hard to find.  Bry's Wrecking in Seattle will buy it from you if no one
else will. Bry had no 2.1 cores when I was last down there a couple of
weeks ago and is always looking for them.

Good luck!

--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT