Potential Problem?

I have noticed lately that selecting first gear when stationary sometimes results in the car shuddering and there is a slight clunk. Almost like I missed the gear. It doesn't happen all the time and as far as I can feel I always manage to fully disengage the clutch.....any ideas...I'm worried there might be an engine or gear box mounting issue, or possibly a problem with the sincro......the last time I had a similar experience was with my Chevette in the mid eighties...this was due to a thrust bearing failure, but I can't see that a front wheel drive car would have the same (??).....
 
My '58 plate's started doing it sporadically for the last few months, I'm on my second gearbox which has never felt 'right' but I'm more inclined to think it's the clutch slave cylinder not being man-enough.

I've read somewhere that this could be the case when the gearbox oil's cold, but in most of the instances i've had it's been after at least 5 miles of driving... the only other thing i can think of is that if Renault didn't follow their own advice and replace my slave cylinder when they replaced the gearbox, then it could have exacerbated the problem.