GTP has issues

I've got a '99 GTP coupe, and recently, it seems to have electrical issues. Sometimes when I turn the switch off, the Retained Accessory power flickers off, then comes back on, the radio is off, and the clock is reset to 1:00. The next time I start the car, it acts like it doesn't want to start, and then after I put it in gear, everything starts flickering and the ABS light flickers on. And occasionally, the alarm goes off at completely random times, the horn will blow 5 times (yes, it's always five) and stop.

It doesn't always do this. It did it for a few days, then stopped for a few weeks, did it again, stopped for three months, and now it's doing it again. Anyone got any idea what's causing this?

First check the simple things like battery condition (many auto parts retailers will check it for free), make sure the battery cables (both + and -) are clean and tight, and check system voltage (again many retailers will do it for free) with a volt meter. It should read 12.6-13.2v with everything off, and 13.8-15.4v with the engine running.

Most of that I've done already, had the system voltage checked with it running, just didn't check it off, the battery's almost new. I need to check the cables, just every time I think to look at them it's after I've been driving a while and it's rather warm under the hood.

Oh, and my sunroof occasionally will open completely with one touch, rather than pressing the switch once to vent, press it twice to completely open.

GM's side post batteries are quite prone for the posts coming loose from someone overtightening the bolts. They are an 8mm head for a reason, they only need to be snugged. I know all about the "forget about it when it isn't doing it, then when it does it's too hot to touch" scenario. Put a post-it on the steering wheel or on the TV (like I do) to remind you to check it when you're not busy and the car isn't hot.