Antenna problems

I am having (I think) antenna problems in my Helio Courier. The radio is a
Garmin 530. The antenna is a forward swept :"V" type mounted high on the
vertical stabilizer. There is a splitter that spilts the output from this
one antenna into two VOR signals and a single GS signal. The NAV flag on
the HSI drops into view and then retracts many times on ILS and LOC
frequencies. The GS seems fine and the HSI D-bar doesn't scallop. This
same 530 works fine in the MU-2 and there are no flag issues in the Helio in
GPS mode, so I don't think it is the radio.

A few questions spring to mind:

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

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)?

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?

Mike
MU-2
Helio Courier