More gear selection issues - advice please?

[GregorJP]

So my car has decided that on the odd occasion, it doesn't like 4th gear when changing up very much and instead of selecting the gear, I get a horrible grinding sound and no gear (much the same as if driving a manual and selecting a gear without fully engaging the clutch). When it has happened, I've taken my foot off the gas, waited a second, selected 5th and it's fine. I can then select 4th again no problem.

Until the weekend, this had happened only 3 times in 5,000 miles and each time it happened on the track, under load and after a fair bit of track time. I assumed it was God's way of telling me to show more mechanical sympathy.

However, at the weekend, it happened just pootling down the road.

I've recently had a new clutch and flywheel, so I can only imagine it's a selector issue, but any ideas would be gratefully received. Cheers.