Long-timer lurker, but first time posting. I have been running a Solidoodle 2 at work for the last year or so. Only upgrades are glass bed + dilute pva glue, acrylic case enclosure, ball bearing-mounted shaft for a filament spool, and attached a passive heat sink to the x-axis stepper (just for kicks, really). We've been fortunate enough not to have any major issues, however there is an odd phenomenon that's cropped up over the last few months:
Whenever I print multiple instances of medium-size and up parts, stringing crops up on concave perimeters and overhangs, along with gapping on convex perimeters. When I take the exact same part and slice with the exact same settings, but just a single one instead of two or three at once, it prints perfectly.

In the attached pictures you can see the stringing on the overhangs, and some gapping on exterior perimeters- "StringyPart1" + "StringyParts2" are of the two parts printed at once; "GoodPartSolo" was printed on its own. The walls are about 3/16" thick, with three perimeters (both internal and external). It's a fitting for 1" PVC pipe. I couldn't get a good picture of the stringing on the inside of the pipe, but it's there. Watching it print, it looks like sometimes on a perimeter the ABS doesn't stick quite right to the layer below as it starts, and the whole string gets pulled around the inside of the circle instead of staying where it's laid down. Why would this happen when printing multiple parts at once, but not with just one part at a time?

Using Solidoodle-sourced white ABS, printing at 210C, bed at 95C.
Completely stock extruder (although upgrading soon to an e3dV6, which arrived last week ).
Repetier Host + Slic3r.
Freshly lubricated rods with lithium grease.

So what's the deal? Any thoughts or suggestions will be appreciated