1994 Ford Escort transmission problem

Hi,

I have a continuing problem with my manual 5-speed transmission in my Escort (1.8L engine, 155k mi). It slips out of 5th gear at highway speed (65-70 mph).

I had the entire clutch replaced in March 2006. The mechanic checked the clutch and said it is sound. He believes the problem is with the transmission synchro now, and the only solution is a replacement ($1,100 quote).

Do you agree with mechanic's assessment, and if so, is it worth putting more money into a 12 year old car with 155,000 miles on it?

Thanks!