Update:
Removed the card, cleaned the TIM off the chip (looked like someone was trying to fill a lake with the stuff) and put some arctic s 5 on there. Re mounted the cooler, tightened up the back holder plate thing, and cranked it up.
Started off pulling 380M at default clock speeds for the card (900 and 1200), and was fine at this for about 3 mins, but the temp was slowly going up and up... When it hit 90, the card then dropped the clocks (mem and main), as if throttled. But, in saying that, it now pulls 320 instead... so slight improvement, but still not right.

Yeah, am starting to think is hardware. My rig is in a large case with 2 (120) fans drawing air in from the front, one from the side directly onto the vid card, and then two drawing out the top(140's) and one out the back(120), so airflow is not a problem.
Have a large Zalman cooler on the CPU, and that sits and 40-50 on full load, have fantastic cable management (there is a huge amount of space around the vid card for nice airflow) and the fans on the card are defntly working. Am thinking the temp sensor on the card is wonky, or something isnt right. Its getting full voltage from the psu (the card has LEDs that change depending on how much power is going in, and these are always on full).

Forgive me for asking the noob questions (feel free to flame / point me in the right direction)

But.... I have an i7 w 6GB ram Win7 64bit etc, and an MSI 5870 Lightning, decent card, but on stock gives me only 260MHs (1.12V, 900Clock and 1200Mem).... Starts off with 380, but then winds down over a couple of mins to 260, fan at 100% all the time. Card is sitting at 96 degrees (3 days non stop so far), but is actually cold to the touch (took side fans off to check its really that hot).

Anyway, I tried to follow the above with flags and edited my config for MSI (both that and CAT are latest), and would only let me drop the mem clock down to 600. As soon as i apply that, thinking its going to run ok, the temp goes from 96 to over 100, and at that point, the pc locks up (when mining, is stable in the os for that, temp goes up about 10 degrees but no issues).
Feeling a bit silly with only 260 and def not sure what im doing wrong....