This problem sounds all too familiar, especially coming from an Isuzu... Hot weather, funny Idle and starting issues and surging RPMs. There was a huge problem with Impulses and Storms with the ECM in the late 80's early 90's. Seems the coating GM uses on the Isuzu Boards and circuits to keep out moisture and dust off them expands and contracts a lot with temp. This will weaken some of the surface mount components and cause running problems.
A lot of the time there would be no codes tripped, or if you had an auto the "code 21" from the transaxle ECU would trip meaning invalid ECM input, which could be the source of the odd TOD light activity mentioned earlier.
I don't know but last I heard the still used the same basic coating on the boards. There was a small Class action suit threatened by Isuzu owners about the same time Isuzu stopped making cars, but it was met with huge Corporate funded Legal Motions and crushed. Now Isuzu does not even admit there was a problem....
Any way, its worth checking out. I had a bad one, and every summer it would act up, I would be almost sick of it around end of Aug and then it would clear up again in the winter and drive fine only to return in the beginning of Summer. Oh well hope this helps, just my .02 good luck.
