Some of the coax is RG-58U, would the newer RG 400U improve things?

Like a band-aid would help a broken arm. RG-58 is just fine for 99.999% of
all installations.



The splitter will supply two VOR recievers. Am I losing something even
though I have only one reciever plugged in? Would a splitter with only
one GS and one VOR output help any? (I doubt it)?

Depends on the splitter. Put a connector with a 47 ohm resistor on the
unterminated arm and see if things improve.



I haven't been up on a ladder to look at the antenna but is the problem
likely to be grounding and/or corrosion? I also have not followed the
coax all the way from the antenna to the radio so there could be
connectors along the way, how much do you lose for each connector?

Corrosion probable. Each connector pair is less than a tenth of a dB, and
not a factor.

Jim