There are three things I can think of. 1, it's a synchro issue because the worn synchro can't engage fast enough when you try to speed shift it. 2, detent ball on the shift rail is starting to wear and catching weird sometimes. 3, your not shifting it right, not likely since you have a shifter but it's still possible.
I would think the first is the most likely scenario. Go way to check synchro action is to clutchless shift while driving normally. If your not sure how to do this I will do my best to internet crash course you.
By the way, you say your clutch is tight, do you mean high up in the pedal? I've seen some cases where the pedal engages real close to the floor and a "weaker" synchro cannot compensate like the others, when infact the clutch does not fully disengage.
