what oil are you using?
Getting this alot lately with people putting synthetic or low viscosity oils in older motors (i mean like more than 50k miles) Has it just started or has it been ongoing? Often starts up when the weather starts to get cooler.
Dont recommend flushing oils, they contain naptha products, which tend to attack nitrile/rubber gasket material, you end up with lots of oil leaks on high performance motors. just put 10 or 15w oil in and try that. When you change the oil, cut open the filter and look for bits of shiny metal to be sure, have a look in the oil as well, Use a magnet to find any small bits, bearings are usually steel/tin/antimony so will be magnetic bits in it if you have a wayward bearing.
Whats it like to drive? feel normal or flat?