Winter mechanical vario reading high

Last year my Winter mechanical variometer may have read a bit
high, but not very much. Today however, on my first flight of
the year, the vario appeared to read 400 feet high in the air and
about 75 feet high on the ground. With the vario reading 400
feet per minute up, the altimiter wasn't doing anything and the
Cambridge electric vario was reading zero sink.

I've searched the r.a.s. archives and found the advice about
zeroing the needle using the adjustment under the faceplate and
putting a trace of nail polish on the needle to weigh it down.

Do these seem like appropriate repairs for this problem?

Can anyone speculate on why the vario might read so much higher
in the air than on the ground? Is there a plumbing problem I
should look for to explain this?

Thanks for any ideas.

Alan