Are you using the inferno build? Try creating a new pilot. Could be corrupted.
As for the 75mm bug I had it happen too but am unable to reproduce it. I tried for about an hour with several different scenarios but never got the bug.
that's funny.... reproducing that error is not difficult for me, it simply happens after every mission where I can equip the 75mm in briefing. And my heart nearly stopped when my trigger go *click* empty on the Drakh in mission 2! haha
Another bug or odd thing (and unfortunately a little mission plot spoiler
I was able to play through the number 6 (I think) mission where the two Drakh ships slice up the Narn station and the Poseidon, right up to the arrival of our buddies, which caused the Drakh Cruiser to ram the Voltoth. It went, I went with it *BLAST* and the game was leaving me with another attempt at this one mission. Now here's the thing, I tried several times but the mission is behaving erratic - to say the least. Drakh fighters simply don't shoot or dead-stop in the middle of space, Voltoth is destroyed by debris from the station (happened twice already) or most of the time the Drakh cruisers exclusively hunt ME and the Voltoth... Poseidon's at 100% while the Voltoth goes booom shortly after the Message to "intercept the Gunships before they bring their weapons to bear" as soon as they are in weapons range for their beams. As trying to dodge those Drakh main beams is hard for my fighter - almost impossible - me and/or Voltoth are usually gone before any further script related to the Poseidon's exitus can even start...
I have a clean install of TBP DVD with the Xmas08 pack mod and DOW3 from your D/L link. Running inferno just fine.... I still have to try it out on my notebook (running the download final version of 3.4), there I was only able to reproduce the 75mm bug but I haven't played beyond mission 2 yet.
I think I will do a reinstall and pray for the final+voice DOW version :-D