Ford: corn in transmission fluid, fush, filter change

Question
My husband and son found corn in the transmission fluid while changing the filter in an 76 Ford pick up.  How is this possible?  We just bought the old truck for our son to fix up and it seems to run fine but is having problems once the truck gets warm staying in the correct gear.  We understand we are probably going to need to replace it soon but had hoped to delay by changing the filter and fluid.  Do you have any advise?

Answer
Corn you say? actual corn or fibres?

Well, a rodent may have been storing it, or a prank by someone.

Corn is very heavy in sugars, so i would get it out of there ASAP.
after that, all you can do is drive it and see if problems develop.

A good fush would be preferred overa drain and fill with filter change,as this circulates the bad fluid from the entire transmission over just the pan.