Won't Go To 4th

Hello all. I've been a reader of this site for a while and finally decided to join up. Found some good info here, and now I could use some more. My 1992 SVX has suddenly decided that it doesn't need 4th gear. In 4th at 2000rpm I should be at 60mph. But I'm not. It's got about 110K - 120K (the speedometer's been broken for a couple of months.) It doesn't get driven as much as it used to (hoping to make it a collector car someday). If I flush the tranny fluid & change the filter, will that help?