I agree with everything janzer said above.
Heat should not be a problem with your set up and the temps I see you posted. And I wouldn't go over your 72w pelt because in my opinion, your set up is working.
As janzer said, start with the little things. Check your CAS setting, change it to 3 if it isn't already.
Are you running an AGP vid card? Try setting it to run at AGP bus speed instead of CPU bus speed. I think the Gigiabyte has this option. That way, your AGP card is not running out of spec and possibly causing problems.
Are you attempting to overclock with
all cards removed from your machine except the vid card? If not, take them all out and try again. Then when you get to max mhz, install them back in one by one. That way, you can find any trouble makers in the bunch.
As far as voltage goes, you will probably have to go to 2.8v to get the system stable. I believe TonForty and Fallguy are both running 2.8v with air cooling only to attain 600+ speeds out of the K6/2's. IMHO, 2.6v just isn't going to cut it for the hIgher mhz/bus speeds. And, if you keep it as cool as you have it now, 2.8v shouldn't hurt it to bad. It won't last as long as a stock 550, but if that's what you wanted, you wouldn't be overclocking, right?
I've been collecting parts to do the same set up as you have and I already planned on going to 2.9v or even 3.0v, but I don't care if I smoke the chip. So, if I were you, I would start increasing the voltage more. I would check the above settings and such first, and yank all the cards out, then try 2.7v, 5.5*110. If it's not stable, go to 2.8v, etc. but anything can happen above 2.6v so if you smoke a chip, don't be suprised. Consider yourself warned! I hate to give advice and have someone smoke a CPU because of it. I'm no guru!
Also, I believe Fallguy lapped his heatsink
and his
CPU cap to help in cooling. This is a long shot, but if you have a hot spot, it could cause problems. And I've seen concaves right over where the core is on heatsinks. Of course, with a pelt, this shouldn't be a problem unless the K6/2 cap is concave.
Are you running some kind of cold plate? I didn't see anything in your post about having one unless I missed it. They really help from what I've read. It's worth a shot!
Hope this helps a little! Post back and let us know how it's going!