2001 Santa Fe with 2.7 6cyl. Began noticing that when at idle for a minute or more the power windows were sluggish (drive through food lines especially). Mechanic tried 3 alternators last one was from the dealer. Voltage drops to below 11 volts especially when AC and headlights are on at idle. Battery was checked and tested fine terminals clean and tight. Did have a fuel pump replaced a while back (failed while on a trip) at a national chain and it had to be redone a few weeks later because it failed again. Radio also turns on and off by itself when driving. Once RPM goes up the alternator puts out between 13 and 14 volts. Now battery is almost always under charged, have to jump whenever I am running several short errands. Did read something about fuel pump grounding problems but really not sure. Any idea what I should try next?

TIA
LSB