Hello everyone!

Before I go any further I would like to tell you that I'm a newbie (only 8k miles done in 9 months) hence my question may be silly for you :P

I would like to ask you guys if any of you have the same or similar issue as mine.

Sometimes when I shift the bike (well, only on 1st to 2nd) I feel a loss of power while I'm accelerating and then the bike downshifts on it's own either to neutral or 1st gear.

I've seen videos of "how to shifting" and how you are not supposed to put too much power/effort on the shifting lever but I've noticed the exact oposite effect with my bike.

I use 10/40w oil which has been recently changed (2-3 weeks ago and about 1k kms done)

Is it me or is it the transmition?

Any ideas?

Edit:

Model XT660X of 2004
The bike has 38.000km (had around 27,800km when I bought it)
Oil change every 5k km or every 5-6 months (If I don't make the kms at that time)