Gears sticking temporarily

Hi hoping you can help me, new here!

I have had trouble on my last couple of journeys with my Astra's gears. All seems fine when I set off (I have a 20 mile commute) but after just a short while I have real trouble selecting a gear. After pulling over and switching off I can select any gear as smoothly as usual. This morning I pulled off road for a moment or two and waited for a minute then was able to proceed without incident. Is this to do with temperature - does the car need to warm up a bit?! It's usually a really smooth gear change so it's really obvious when it becomes sticky but as I say it is only tempoary and seems to "cure" itself pretty quickly. It's just a bit hairy driving thinking it's going to seize up again! I can select all gears with the engine off easily. Any ideas?

Thanks Paul