Need help cutting MP4 videos Virtualdub-style

After some research I have tried a couple methods but neither seem to work well. I’ve added filters to Virtualdub’s plugin folder which allows me to open and edit MP4 files, but it can’t save in the original format since VD was designed for AVI. The final result (let’s say a 5-second AVI) is almost the same size as the entire original 30-minute MP4. Not very efficient.

Alternatively, I downloaded “AVI Demux” which lets me open MP4 files without serious issues, but I get a warning at the beginning about losing frame accuracy, and either the program crashes at random times, or I finally manage to cut what I want (albeit sloppily since the cutting function wasn’t agreeing with me) but the final product is glitchy when playing back. Poor framerate, sped up video, no video, desynched audio, etc. Sometimes the part of the video I wanted isn’t even there. I understand there are complications because of “B-frames” and the like but I’m not asking to do anything except cut the damn thing.

I know I can just convert to AVI first and Virtualdub the heck out of them without problems, but the files are often large and I don’t really want to lose quality in the conversion process. Any ideas?