Science lesson:
Steel is almost elastic. Elastic means when it is stretched then released it returns to it's original shape. Steel will stretch, but doesn't return to it's original shape, it will be microscopically longer.
On every power stroke, every time the engine fires, it stretches the head bolts, over time the head bolts get noticeably longer... which is why you re-torque your heads. (YEAH, I know YOU don't re-torque your head bolts, but you're supposed to).
Every time the head bolts stretch, they lose a little elasticity and get harder (work hardening). When they lose elasticity, you lose clamping force.
So, long story short: head bolts wear out, and it doesn't fix them by over torquing. Over torquing them will over stretch them, necking them down, making them weaker, making a bad situation worse.
The FIX is to replace them. The EX was a CHEAP bike, built to a price point, and those head bolts are just this side of mild steel, as in "not very good from the get go".
I just did a GS1000 motor and three of the 12 head bolts were 0.100" longer than stock. They are a stupid design where they neck down to 8mm in diameter to facilitate oil flow up to the head. They got replaced with 10mm tool steel head bolts I made ... so the clamping forces hold the head down, not stretch the bolts.
I am under the impression torque + angle head bolts are stretch to yield, where the bolt "stretch" is what gives you clamping force... and the bolts are one use only. I haven't seen a bike with stretch to yield bolts, and I know for certain a EX doesn't have 'em.
Torque is a lousy way of measuring clamping forces... cuz you're not measuring clamping forces, you're measuring the "gall" in the nut to bolt and the nut to head surface connections. A much more precise method is to measure how much the bolt stretches.