I live in a two story house and replaced all water pipes with Pex about 8 years ago, installed a tankless water heater 3 years ago, and have a 7 year old water softener. About two months ago, the hot water pressure changed dramatically. When the hot water is turned on at one faucet, it runs with a little less pressure than before, but after about 5-10 seconds it greatly reduces. If another faucet is tuned on to hot water anywhere in the house at the same time, the hot water goes down to a trickle. Sometimes when this happens, if the faucet is turned off and then turned right back on again, the pressure will be a bit stronger, but not normal. Any idea what is going on?

after about 5-10 seconds it greatly reduces. If another faucet is tuned on to hot
This tells me that you have a volume problem. What's happening is the water builds pressure and volume behind a partial blockage. Then you make a draw. For a short time the pressure moves the water past the clog. When the clog won't let enough volume past the pressure falls off. I can tell you what's happening but I can't tell you exactly just where the clog would be found. A good place to start looking is the tankless heater. Good luck, Tom