I AM CONFUSED (NO COMMENTS PLEASE )

drove the car today. felt rougher than yesterday.
called a mech shop to take it in. drove the car there and i could feel under acceleration that the engine was clearing as i drove.
when i got there it was nice and smooth. tech felt i should wait and get it back when car is "bad" again.

it does not seem like a vibration from any mountings but rather the feel of the fuel or spark not getting through cleanly.

- how do i check the coils?
- how do i test the injectors to see if the right voltage is getting to them?
- i saw a post in the archives about somebody having problems and it ended up being the battery??

ANY HELP WOULD BE APPRECIATED.....