ZSU Controller reported chronic transceiver malfunctions resulted in serious unsafe air traffic situations.

Narrative

I was instructing my trainee on r4/6 when he asked me to take over. I took over the sector and it became extremely saturated and should have been split off. I was barely trying to keep my head above water with the amount of aircraft I had to deal with especially since about 4 or 5 of them had to descend from the high 30s down to 130 crossing a very crowded departure corridor. With all this going on; frequency 134.3 is an absolute safety hazard in the eastern corner of the airspace. It takes you multiple transmissions and a bunch of toggling between mains/standbys to be able to talk to pilots.It didnt help that the some of the aircraft coming in over OBIKE needed a route change due to a flow from TNCM and they also needed a quick descent. Most sessions dealing with that OBIKE to NEYDU/DAWIN to LAMKN corner of the airspace is a huge pain due to the completely garbage frequency; but in this situation where I was barely keeping up with the traffic it was a miracle an accident didn't happen when you have a bunch of aircraft converging in one corner while it takes you 5 transmissions just to radar identify someone in the opposite corner. I'm not going to hold my breath and expect the frequency to be fixed anytime soon.The airspace needs to be redesigned. That eastern edge up to the R2 boundary should just be given to ZNY. I would suggest it just get changed to R2 since 118.15 works perfectly in that corner but that would add unnecessary workload to a sector that's already the busiest in the facility.