How can I fix my 1997 Ford Mondeo Automatic?

Around two years ago, I replaced my automatic gearbox with a second hand one after the original failed. It has worked fine up until about a week ago. Now it is very reluctant to change into higher gears and I don't think it ever actually reaches top gear. The green 'O/D Off' light flashes after it has been running for a few minutes. I do not know how to check the fluid but I was wondering if contaminated fluid could cause this problem. My wife called into a local garage where they checked the fluid level, which was apparently ok. They said they did not deal with gearboxes but they felt it was something to do with the Overdrive, which would require a new gearbox. Due to the age of the car, I will not be replacing the gearbox again. Do you have any good news for me??