M3 E46 SMG Gearbox problem

[gmtc]

Dear all, my car has developed an intermittent gearbox problem in the last two days wherby it gets stuck in 2nd for a period of time, not allowing me to change up in manual or auto modes. After 20 seconds or so of trying it will change up to 3rd-4th etc and then function perfectly for the rest of my drive. Sometimes it will struggle to get out of 2nd repeatedly though it will usually change up after a bit of trying.
Any ideas where my fault may lie? The first real problem ive had with the car in nearly 4-years of ownership.
Many thanks in advance, Graham.

[gmtc]

Thanks all and Mr T-Bagger
The symptoms start after driving for half a mile or so with no apparent problems. Once i stop at lights or in traffic and then move off i shift from 1st to 2nd at normal revs however when i try to shift to 3rd at normal revs I get no response - stuck in 2nd. The yellow cog? light comes on. Then, usually after trying paddles and also the gear knob for 10 or 15 seconds the gearbox will shift to third or sometimes 5th. Changing down has never been a problem. After a little while of this often all symptoms can go away. Its intermittent as at lunchtime I took the car for a drive and experienced no problems whatsoever despite stopping 2 or 3 times. The yellow cog had disappeared and did not reappear. Its been like this for 2 days. I've had suggestion of low gearbox oil?? (its due for an oil service).
Thanks you guys for your advice!