'98 weird electrical problem

[mtglick]

Hi, guys--nice place ya got here!

I'm having a problem with my '98 SL500, and I don't exactly know where to begin. The problem is electrical (probably), and intermittent (of course). I'm not even certain if this is one issue, or multiple ones, but what I do know is that I'm afraid to drive it any more--nothing makes sense and I can't just keep throwing parts at it.

The history: The car has about 125K on the odo, I've put about 10K on it since purchasing in 2007. In the summer of 2009, we started noticing a small starting problem--it would crankandcrankandcrankandcrank for a few minutes, then if you walked away for about 10 minutes, it would fire. This got progressively worse over a few months, until finally it refused to start altogether for a few days and sat in a parking lot over a weekend. On Monday, it fired up as if nothing had happened. I drove it home, and on advice, replaced the CPS. Engine fired right up. It ran great for a few months, but the problem started up again, needing a few minutes to start after refusing.

Then it had a big problem. While running, the CEL came on, lots of lights flickered, the car lost power, and ended up being carefully driven to an independent's shop. Two coil packs and a brake job later, the car was returned. The independent noticed the same refusal to start, and thought that the key might be the cause, but the car didn't throw a code, so he couldn't confirm it. Parked it for a while, tested it every so often, always started up. Somewhere along the line I misplaced the alarm fob, so I started using the straight key. No appreciable change in behavior

Continued using the car, with the occasional starting issue, and on the way to work one morning it died cold while running. Nothing, no starting, no action at all. Didn't crank. Car had power, battery was OK, LED odometer worked, headlights worked, but the rest of the electronics were out. Parked the car, and came back later, the power was back, car would crank, but no start. Did that for a week, sometimes the power was on, sometimes not, finally it started and I drove it home. Once home, it died again and refused to start, sometimes it would crank, and sometimes not. I started to try to isolate symptoms, but they were intermittent and I couldn't put my finger on a root cause. Found the alarm fob, tried to use that instead, no change. At that point it seemed like there were two separate problems--a cranking no-start, combined with some other larger electrical problem. So I parked it, covered it, and ignored it. For about a year.

Last weekend, I decided that it was time to revisit the Benz. Uncovered it, hooked up a battery charger, and left it alone for a day. Came out the next morning and tried it, and the damn thing fired right up. No problems, other than running slightly warm at first, just sitting in the driveway. Slight valve tick that self-resolved.

WTF?