Try an IVD/port/cc cleaning and a bottle or two of PEA/PIBA based FI cleaners at your next fuel fill ups. I'd also clean the TB and the AFM/MAF if equipped. Might also need to verify that temp sensors are reading properly.
Also, with that many vehicles, if you are not running all their fuel tanks empty every month, then you need to use a fuel stabilizer. I have daily drivers, summer only driver, and hardware store drivers. Gasoline could be topped off weekly in 2 off the vehicles, monthly or even quarterly in others. The latter get dosed with StabilMarine360, Lucas Safeguard, or StarTron at every fuel fill up.... just like my lawnmower, tractor, and generator fuel containers.
Being that these are older vehicles, have the spark plugs, air filters... been changed? How old is the coolant, thermostat, and radiator cap?
Otherwise, if you are having problems with regular from 2 stations, have you tried their midgrade? If the issue doesn't go away with their midgrade, then the smart thing to do is simply avoid those stations.
Shell around here tends to be pricey. As such, it volume of business is less than other stations. Volume means gasoline age is greater. Octane degrades with time at the station and in your tank.
Btw... nothing REQUIRES 87 octane. Fuel octane is a recommendation. And, most manuals state "87 or HIGHER". In this situation, you might need higher with those stations.