Puzzled!

I’ve got a right hand drive 2007 Cayenne S, which has done a little under 65,000 km to date. It had a Porsche major maintenance at the very end of 2018 (which replaced the battery and replaced two gas pressure springs, but was otherwise fairly straightforward). About eight-ten weeks later, I got a yellow system faulty warning, and took it in for a look. At that point, the car also looked a little tilted (high at back left and low front right). The garage recommended replacing both the front air springs for suspension (which is not a cheap solution...). The car sat for about three-four weeks, as I was then travelling, but when I got back I noticed that I didn’t have a warning any more. Long story short, I’m now sometimes getting a warning after driving from anywhere 2-15 minutes, but I’m also doing trips of 15-20 minutes without one. Car doesn’t look tilted any more, and I can move the car up and down (and be in sports mode when there’s no warning light). Car feels to be driving OK, but there does seem to be a battery problem (an initial chug on startup). Looking at the forum here, these warnings are not always what they appear. Any wisdom before I go a costly route (which the garage doesn’t seem fully convinced of, as an aside)?