It could be any of the things you mentioned. What's the weather like in Herndon? You may need drygas if the temp is below 32*. Most of the knowledgeable people on this website only know about american made V8s. You need a mechanic that works on imports to eliminate the possibilities one at a time. If it's going in for service this week they should solve your problem anyway. If you have any questions regarding swapping in a Chevy V8 I'm sure we could be of more help.
AL