I'm with Josiah , you're describing classic tight valves symptoms . no one likes to do this important job so they al say " oh I checked them , they're fine " ~ every time I offer to bet $5.00 they're tight the story suddenly chages .
Valve adjust , set dewll , set timing and it'll be fine .

-Nate
There is no problem so difficult it cannot be overcome by generous application of brute force & ignorance