put a NOID test lamp (or LED+Resistor) on the harness connector terminals and see if you get a pulse when the engine is running. If there's a pulse -- injector is getting a good signal. Also easy to test the coil pack for spark -- if that comes/goes, could be the PTU or more likely a problem with the PTU wire harness.
My bet is the injector is starting to fail - coil wire inside is starting to break and contact comes/goes due to engine vibration.