Anyone else facing consistency problems with TTL?

Much of my photography involves photos of rock bands. Quite often I use two remote speedlights (SB-600 and SB-800) with the D300's own flash as a commander. In other times, I might mount the SB-800 and use it as a master.

The big issue is, I cannot trust TTL metering anymore. I am one step away from switching to Manual - with all the extra effort this will require - simply because of huge inconsistency issues. Often I realize I have to use up to +2.0 flash exp. compensation to get what I want - this is an insane difference. It feels as if the TTL system strongly protects highlights, I have no other explanation for it - because I get extremely underexposed shots when using TTL (both from the onboard and the remote flash). It is not a matter of recycling time or anything like that. The worst part is, every now and then the exposure goes actually way over, indicating that TTL metering would've gotten it right without the compensation.
I am completely baffled by this. Anyone else having faced similar issues?

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