Before getting a local plumber involved, I'd very much appreciate some thoughts on potential causes of my issue. Thanks in advance.
1. When taking a long shower, there is an almost split second sudden drop in pressure (as if someone turned OFF the water supply then immediately turned back on, but even more brief than the time this would take). This is similar to the effect of flushing the toilet in the old days and having a split second of low pressure (but I don't get, for example, all HOT or all COLD, it is ALL the water going off like holding your hand in front of the spray for a second to simply "stop" the spray.
2. More details.
o This happens only once every 7 to 10 showers, not every time. When it happens, it doesn't feel like all cold or all hot remains, it is more like all water stops, but so briefly that the shower head is never completely off (that is, it kicks back in almost immediately). Otherwise all pressure in shower and rest of the house is very strong and consistent.
o If I try to create the problem by flushing toilet, turning on other taps, etc. I can't. That is, I do NOT lose any shower pressure from flushing toilets, running other taps.
o This is an old house, but all the plumbing was done from scratch about 2 years ago, including line to the street meter, new hot water heater (electric), gut renovation of bathroom/shower, new feed lines, fixtures, etc.
o Problem only appeared in the last month or so, but it is so intermittent, that it could have always been around and simply didn't happen while showering.
o I haven't noticed any loss of pressure when using kitchen or sink in bathroom, but then again, I'm never running these taps for as long as I run the shower.