Thanks for the input, guys. The vacuum leak idea in particular jibes with past experiences. I had an immensely frustrating day today trying to change the fuel filter. Didn't really think this was the problem but thought I should change it on general principles. The one I bought at AutoZone turned out not to fit, though--it was at least 1/4" larger in diameter and would not slide into the clamp, let alone the plastic sleeve inside the clamp. Went running around to other parts stores and found only one other fuel filter in stock, a Purolator, also too large. Finally just re-installed the old one and moved on to do an oil change and replace the serpentine belt, two post-purchase jobs I didn't want to put off any longer. That went OK except that the belt tensioner bolt ran out of extension before the belt was tight, so I had to jury-rig a way to tighten the belt adequately. Is it just me, or is basically nothing easy on these cars? (Maybe you can tell it's my first experience with MB.)