Nobody knows what sort of interference, except for the guys that no
longer work there.

I'll try clearing it up:

Tektronix 475(?), sitting on plywood, ontop of a metal table.
Scope is plugged into a power strip that is shared by another power
strip, a computer, monitor, and hub. That power strip gets it's power
from another strip (I believe they are all NCR surge protectors). These
strips plug into the same quadplex outlet, which is full.
The table is not grounded, and sets on a concrete floor.
The shop has walls made of cinder blocks, and the roof is a fairly
plain metal type, with steel lattice work.
The cell tower is on the same property, about 100 feet away, and I
suppose 200ft tall, but I don't know for sure.

Tomorrow, if I'm not too busy, I'll go over the scope with a grounded
fine tooth comb.