1994 Honda Accord overdrive shift problem

I have a overdrive problem. The transmission shifts fine but when i get up to highway speed it is reluctant to shift into the top gear.It does eventually but if i keep at a steady speed [say 65 to 70mpg] it will downshift and shift up again.Ive had a similar problem like this many miles ago and i cured it buy changing the thermostat.i did so again but this time no luck.The fluid is full and clean and i dread the expense of transmission work.What do you think.