I can think of two things that might cause the condition.
First of all it sounds like you may have excessive endplay in your mainshaft assembly. When you downshift the thrust loading of the mainshaft is causing the mainshaft to shift forward in the case and the second gear clutch teeth move away from the synchro sleeve, hence the slipping out of gear. This is most likely due to worn bearings from high mileage. It isn't going to go away on it's own and will get worse. It will lead to more problems and possibly catastrophic failure if left unattended to.
A second possibility is it could be a bent shift fork and/or worn shift pads. In any case I would suggest a basic rebuild before something expensive breaks. you should be able to get a basic rebuild for something around $350-$400 if you take the transmission to them. Sounds like a good time to take care of any clutch concerns while your at it.
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