The front end, i.e. 3SK74, could have been damaged by RF overdrive / indirect lightning strike / static - take your pick.
If it is the latter two, then a unun with a ground connection may help.
Some suggested PN diodes, e.g. 1N914, BAV99, Ge, etc., but they will also act a mixer to bring down the IMD performance. A PIN diode limiter would be a better protection for RF overdrive as it is more linear than a PN diode. The package and configuration are important as you want to minimize loss at higher freq. A good reference:
http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an1050.pdfAs for the suggestion to use a silicon transient voltage suppressor (TVS), most of them don't have low enough capacitance to be inserted into the RF path.