Feels like i am towing a boat!!!

I am driving home from work and i am cruising about 60-65 and i feel the car driving like it towing a boat sluggish and just poop like. Anyways the car popped out of 5th for the first time ever. So i put it back in 5th and within 10 15 sec did it again and kept doing it all the way home about 15 times total. Now i read in the sticky posted by chrono404 about DIY troubleshooting and it seems like i will be replacing the gear set, synchro sleeve and the synchro ring for that gear.
But what would cause the car to FEEL like i was towing a boat? Also how is it possable that the tranny just took a dump in that gear that fast because it never did it before. I think it did it twice in 4th but thats it. Any ideas!! what should i look for before settling on getting the tranny fixed?
But what would cause the car to FEEL like i was towing a boat? Also how is it possable that the tranny just took a dump in that gear that fast because it never did it before. I think it did it twice in 4th but thats it. Any ideas!! what should i look for before settling on getting the tranny fixed?