charging/discharging problems

[mick brumby]

I've got a 2005 Chrysler 300c 5.7 with an annoying problem,sometimes the battery runs flat with no warning at all.It is not the battery at fault,it can happen on a long motorway trip,you stop for a coffee after a 70 mile drive,come back to the car after 20 mins. and the battery's flat.the AA mechanic tells me that the alternator is OK.The car may be OK for 2 or 3 months (used 7 days a week)and then one morning I get in the car to go to work, and flat battery.When the car is at rest with everything switched off the current drain is about 3 amps,but after about 5 mins. it drops down to about 90 ma.The alternator is original, and has done 139000 miles.the battery is 2 years old,but a new one has been fitted,but made no difference.
any ideas?