tek 453 x or y issues?

[m3vuv]

hi all,ive a tek 453,im trying to use an octopuss with it,when i first put it in xy mode it displays a dot as it should do,then shorting the octopuss probes together i get a verticle line as expected then after a second or 2 it reverts back to just a dot on the display,when i tap the scope i can get the trace back to normal for another couple of seconds,would the problem be with the x or y circuit?,it only plays up in xy mode and fine otherwise,i dare say its a socketed transistor dirty but not sure where to start looking,cheers m3vuv 73