ok. occasionally i'll go to start the car, and it takes a few rounds for it to fire off.
other times, it won't fire at all, something just spins when i hold the key on start.

and other times, i'll turn the ignition on, and it'll turn over and grind exactly like it would if the car was already started and you turned the switch to start.

not exactly sure whats going on, but i'm afraid i'll get stranded somewhere sometime and not be able to get it started.

i've had a shop look at the starter a couple months ago, and they rebuilt it which stopped the grinding for a bit, but now its back.

anyone have an idea as to what could be wrong?

 

Start up issues are either a mechanically or electrical issue. Since u have mentioned that the starter has been rebuilt once, it is possible that the starter has weakened to the point where it can no longer turn the engine over. This would be an mechanically issue. In which, replace the starter with a new one, if u have the cash.

There are a number of electronics that are associated with start up. However the electronics that are the cause of much of the problems with start-up are the camshaft position sensor (CPS) and crankshaft position sensors (CKPS POS and CKPS REF). SEARCH. There are alot of threads about yur specific issues and about these sensors as well. You will want to diagnose these before replacing. From autozone i think the CPS is 50 bucks, the CKPS POS and REF are 70 and 90 bucks respectively. Make sure to check their electronic harnesses too.

 

are there any CELs i would start with adding extra grounds then replacing the starter(it sounds like you need another one) then test sensors

 

I had similar issues with my maxima about a week ago, replaced the starter and everything's been running smooth since... I would take your rebuilt starter back to that shop and have them test it.

 

+1 on the starter.