Okay, so did this problem appear right after installing the new clutch? Indicates improper installation and/or adjustment if that is the case.
You state at first the transmission has a problem, but you do not specify WHAT problem. (only that it occurs between first and second)
Then, you state that it doesn't jump out of gear, eliminating one of many possible problems.
So, what is the EXACT problem? Does if jam? Stays in previous gear and won't change? Won't change up, or down? Both?
I'll pass along your thanx to our son who is headed to Germany in December for 2 or (hopefully) 3 years along with his entire family, car, motorcycle, dishes, TV, and dogs.