There are various discussions on forums on different performance of different TV sets and tuners, the signal strength shown are relative and based on the formulae used by the manufacturer.

Search for "lg tv astc tuner problems" and see issues in gneral.

However, if you are right that a direct connection from antenna without amplifier to your LG displays chn 5-1 perfectly, then it is NOT a multipath. The problem could be in the amplifier ( got noiser or something else, but I think not), a connector on jumper from amp to LG (Not likely), more of a rf signal level slight drop to the LG TV from new leaves on trees or new condo somewhere along the rf sight line.

I would still try a pre-amp with passive splitter, 3 way passive splitter ( 5 to 2000Mhz) at dollar store and a $50 pre-amp.

Please ensure that the LG TV does work solidly directly connected to antenna, before you do anything else.

Good Luck

PS: Some days I still have a solid 50% on chn 29 and it breaks up or totally black, then comes back, could be gone for minutes or days. My TV only needs 20% signal showing to recieve a picture. Weather multipath and reflections off of condos south of me