I own a 98 black SVT with 52000 miles on it.
I have the same problem, rpm drops to below 500 at stop light, then shoot up to 1000 then down to normal ~600.
(I know my idle is usually lower than 750 +/- 50 spec)
Since someone here said it is related to the heat, I think it's somewhat. However, I recall that mine has been doing this regardless how hot the weather is.
I'm thinking it's more related to IAC valve. I cleaned it with TB cleaner once, but it seems to be gotten worse after claeaning. So i'm planning on replacing IAC valve soon. I'll let you guys know after replacing the IAC
