Missing fifth

My vrs se loses it's fifth gear... always between 6 and 10 miles from home, after a cold start. You try to select fifth and it just ain't there (the gear lever is baulked – it'll go into sixth or back into fourth, but not into fifth). It soon clears. After a mile or two fifth reappears, but I'd rather it didn't happen at all. Has anybody had/cured this problem? The garage can't replicate it, so it doesn't exist! It like to get it fixed before the warranty runs out...