Welcome, Gary!

  There are several possibilities, but my first suspect would be the antenna and/or the "sharing" of that antenna.  Does your AIS have a switch inside of it, or how are you sharing that antenna?  The normal way is to connect the antenna to the switch, then the AIS and VHF Comm radio have leads from the switch.  Many newer AIS units have the switch inside, so the antenna connects to the AIS, then there is a cable from the AIS to the VHF.  If there is an external switch, does it have power applied?

  Try removing the AIS and the switch from the situation by running the antenna direct to the VHF Comm radio - if that improves reception back to "normal", then the AIS and/or switch is suspect.  If it does not, then try substituting the antenna with (for example) a short piece of wire (don't try to transmit!) and see if your WX receive improves.  If it does, then you clearly have a bad antenna or coaxial cable (I would replace the cable first - in my experience, these are 80-90% of such problems.)  

  If the reception still isn't good, then you probably have a radio issue - but I think it unlikely that both your AIS and VHF Comm radio would develop poor receive at the same time (unless you took a lightning hit!)

 GL!

Hartley

p.s. - I see this is a VERY old post - my apologies..