this happened in mine,now its jumping out of fifth.sometimes its just a bearing that goes on the mainshaft,one way of finding out if its a bearing or if your gearbox is rightly fu**ed is to drain of about a half cup full of gear oil,if its full of metal particles then your box is cooked,if its clear then youll probably get away with a syncromesh ring or a bearing.unfortunately for me my gear oil was the former,i have a reconditioned box on the way