Boy do I remember those days. I started in RC with the TRX engines. They make great power when they run right, but that perfect tune is a bit elusive. I'll let you in on a few things that I found to work with them. First off, they leak air. EVERYWHERE. I never had one that didn't. Seal it off well with RTV before you try anything else. Carb, backplate and bearings are the problem areas. The needles get loose and won't hold a tune after a while. A bit of teflon tape around the threads works pretty well to tighten them up again. I changed to OS A3 plugs. They just seemed to work better, longer. If you are still using the electric start, you can file off the post a little so the starter wire still fits. Tune the HSN with the smallest increments that you can make. It seems like the TRX is more sensitive to tuning than any other engine that I ever used. Keep the fins clean. Drill vent holes in the body. Different fuel brands and mixtures can make a huge difference in tuning and heat. I found out the hard way that they don't really like the low oil racing fuels.
If none of these fixes work, throw it away and get an OS!
That's actually what I ended up doing. I don't run any TRX engines anymore, but I keep one to remember the good old days!