Shaking the bolts loose

I have an 89 Electra Park Avenue with 230,000 miles. I got the car after my mother in law passed away in October. I recently replaced the EGR valve, coil, fuel pressure regulator, PCV valve, intake manifold gasket and added seafoam to the gas tank. The plugs and wires were replaced about two years ago along with the fuel filter. It will run fine most of the time but sometimes will suddenly start to run very rough and shake hard if I try to give it more gas and will idle rough after this. It will do this at any speed with no warning. I do notice the RPM’s drop just before it starts to shake as if the transmission has shifted gears. It also seems to run fast; that is it will almost maintain speed without even touching the gas. I have to keep my foot on the brake going up my street to keep from going too fast. Any help would be appreciated.

You may have an Idle Air Control (IAC) problem, or vacuum leak. You can remove the IAC & try cleaning the pintle & seat with carb cleaner. In addition, check TPS & verify voltage @ closed throttle & WOT. It is easier to use a scanner for this purpose, but the wires can be probed for voltage, if you don't have one.