Gear selection problem in heavy traffic - Mk3

I have an intermittent problem with selecting gears in my 2008 1.8 5 speed MX5 (106 000 miles). This only occurs after driving in stop-start/very slow traffic and especially on a hot day. The gears become harder to change over a period of 10-15 minutes, until I have to force them in. I've noticed that after parking up for a while, say 30 min, the problem is particularly bad when I return and reverse gear is almost impossible to select.

I am familiar with the common problem of a notchy gear change when cold for some, but this seems to be something else. In normal free-flowing traffic, the gear change is good, except the occasional grind between 1st and 2nd if I try to rush it at higher revs.

I've changed the brake/clutch fluid and bled the system, which I thought had fixed it until I got caught in traffic today and the problem returned.

Any ideas?