[TenTec] Omni IF gain and heat

[JACrux]

I notice that if my Omni-V.9 has been on for any length of time, the gain 
seems to fall off. The is especially noticeable on the upper HF bands and I 
have to turn the RF gain way up. 
This is probably something changing as it gets thoroughly warmed up, but has 
anyone any idea what ?? Do I need to install an aircon system, or blowers, as 
suggested for an Omni-VI ?

John  G3JAG