Do yourself a huge favor and ground your amps to a strut mount mounting bolt anduse a star washer under the lug and a flat wasner under the nut and make sure that they are tight and you should eliminate a lot of your problems. Yes I would put spacers under the amp so you get some air flow all around the amp, should help with cooling.
The following was taken directly from Hifonics Website
It is highly recommended that the amplifier be mounted to a board of MDF or other solid
structure using the 4 mounting screws provided. Avoid mounting the
amplifier to metal as this can introduce noise and other unwanted issues. When mounting
the amplifier, ensure that it is mounted HORIZONTALLY, as shown in the diagram above, for
optimal heat dissipation. Mounting amplifiers to speaker enclosures is not recommended as
this can cause damage to the amplifier components.
