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Hits in CRT

I have a problem that has been driving us nuts here for a while. Of 
course its an intermittent problem so it has been very elusive to 
catch. We are getting 10 to 20 line "hits" in our CRT in our HD Rank. 
These hits when captured on a still store or tape are just a drop in 
luminance ( aprox. 10IRE in a bell shaped curve looking in the field 
mode ) in positive and a bright up in negative, The picture is 
completely in focus underneath the dim area and there is no geometric 
distortion to any of the picture. This led us to belive that the 
cathode or grid was having some problem. We have noticed that when 
the Rank is cold that ( turned off for a while, or we bring down the 
temp in the machine room) that these hits seem to appear once or 
twice per 5 minutes. We have replaced: CRT's, High voltage PS, CRT 
socket,CRT blanking, Backed down the High voltage to 25KV ( runs at 
30KV normally ) removed the focus board, removed the pincushion board,
 ran the back end of the CRT on its own independent supplies of 
(filament, cathode and isolated the grid from the blanking signal) 
replaced the scan rack frame ( not the cards ) replaced the main 
power switch box ( beam current control ) and removed the rubber 
insolaters in the Mc Murdos. The only thing left seems to be the 
cradle or convince Dave Walker to make digital scans for High Def.
When you hang a scope on the grid or cathode you can not see any 
disturbance. This one has me baffled. Any help would be appreciated.

Howard Lukk
IVC, Burbank