Dunno Quin, but I've experianced this myself and as my m8 has a tyre bay I got him to check my tyre out, he couldn't find a puncture or leak at the rim even though he applied a little more air than standard.
This wont make much sence to your intermittent deflating but are your wheels perfectly round? it may be buckled letting air out, put the wheel in a massive bucket of water and push on the tyre walls and see if bubbles come up, the chance of you having a bucket this big is not great but some tyre places do
Usual reason is corrosion of the edge which pushes the bead away from the rim. Do as Jack sez, whip the wheel off, lay it flat and put some soapy water around the edge. If its does show a leak then you'll have to get the tyre take off, wire brush/rub down the edge and tyre resealed.
Shoudln't cost much more than a tenner.
Other than that, it may be a faulty vavle or local kids playin wiv your head
Get your tyre bay to wire brush/clean the inside of the rims and apply a sealant prior to fitting the tyre.
Some tyre bays don't both cleaning the old sealant from the inside of the rims, this can sometimes cause the problems.
It could also be the valve, you use to be albe to replace the inside of the valve, I don't know whether this is now possible.
Try deflating the tyre (this can blow out any grit trapped in the valve seat) and then re-inflate.
...actually, the faulty valve idea COULD just be right...a few times
when I've re-inflated the tyre, after I've removed the air-pump, the
valve has stuck and the tyre has started to deflate again so I had
to put the pump back on and remove it to stop it jamming
...did once get it repaired fer £6.50 in Brum...the bloke said it WAS
leaking air from the rim but he said he'd fixed that...maybe he didn't do
such a good job after all
Actually I had a tyre losing pressure and got it sorted on Friday (since I didn't want to drive up to the Lake District on it)
It was just a bit of corrosion, they wire brushed the rim down, and applied liberal amounts of rim sealant (fnar fnar) and it seems fine now. Although I have loads of orange gunk all over the alloy now
Found a nail in my left rear on Sunday now
Doesn't seem to be deflating though, will take it to the tyre place tonight. Hope it's repairable, I've only had the tyres for a couple of months. It's only about an inch from the sidewall
Bazza ...............those "guidelines" for puncture repairs were brought by the National Tyre Federation a few years ago to help all the fast fit outlets sell more tyres(trust me i used to work in one)
Bet yer a quid they say its not repairable as the edge of the patch will overlap the sidewall and will peel off causing a blowout or similar
Yer best bet is to go to a local friendly garage and get them to repair it as 99% of the supposed unrepairable tyres are repaired and sold by the tyre fitters to there mates (trust me i used to do it )
My local (very small) tyre place use the push in rubber plugs that seal from the inside out so no chance of them "peeling" off. They will repair anything except sidewall but to be honest I don't know why that is as it should repair ok (imho!).