88 300se

it just started to do it .. when i drive slow i cant notice it ..but when im on the freeway at a steady speed i can feel my car back off for a split second .. i watched the rpm gauge to see if it dropped and from what i can see it does drop any at all .. maybe it just happens so fast that i cant see the drop just by looking at the gauge ..
i can accelerate and it picks up speed like normal.. just replaced plugs a few weeks back. IAC was recently cleaned. im wondering if my fuel pump is getting spikes of voltage loss or possibly my coil or plugs are bad .. i did notice when changing my plugs cylinder 4 plug is a bit loose when putting on and taking it off of the plug .. doesnt seem like enough to be a problem .. i replaced the ditributor few months back

i dont know if this is related to the same problem but i came up on a stop sign and kinda stopped quickly and my engine stalled .. it hasnt stalled since but every once in a while it seems like its struggling to get back to a normal rpm idle .. (kinda like on an older carb. car when u come up to a stop and the car hasnt down shifted and it struggles a bit till it kicks down)

where do you think i should start ?

im gonna replaces wires.. how do i check coil ?

could this also be an injector problem ?