Rough Idle, Occaisional Stall and more.

Hey guys, new to owning an RX8 but not new to having issues with vehicles lol, so my issue is when i start the car it takes a while to crank over so my initial worry was the battery, surely enough the terminal definitely needs to be replaced, my multimeter took a reading of four volts from the battery while off which doesent seem possible? So when the car does start, usually will, just takes a few seconds of trying, the temp guage will climb when it reaches around quarter warm the RPMs will Fluctuate from approx 1200-750 once its warm it will hold a steady idle, while i drive it after a bit ill notice the rough idle come into play again, it will fall from around 900 to 750 sometimes as low as 500, it will usually recover, but today it actually stalled on me in traffic for the first time while stopped at a stop sign, i was able to crank it back over and make it home. Ive also noticed an odd sound coming from my engine bay when i step on it and reach 6-7000 rpm quickly its almost like a fluttery noise sounds like my intake is making it but ive checked everything on it and it looks fine.

MODS:
Injen Cold Air Intake - The fluttering sound came shortly after installing this ive cleaned my MAF and reset my fuel trims and my shaft sensor still to no prevail.

When my car does start its pretty weak, but ive replaced the plugs with NGK laser iridium plugs aswell as the ignition coils so that cant be the issue, am i looking at compression failure? My 8 is a 2005 MT, has 109,000km on it, i do all my oil changes on time if not early, but i do not premix, i drive it like it needs to be driven, hard and fast.

I have a can of sea foam sitting ready to go but i feel if its compression loss its not worth using if ill just have to replace engine? I am under warranty