First thing to do is get a scope and start probing around. If you're 
measuring with a meter, it wont show the peak voltage.

I recently debugged a nasty switching-regulator problem in my wristwatch, 
only to discover the bench supply *plus the inductance of the testleads* I 
was using had tons of noise, so check all around the switching regulator as 
well.

Also be aware the feedback path to the regulator has a low-pass filter with 
a cutoff frequency around 160Hz (assuming I did my math correctly for 
Fc=1/2pi*R2*C2. You can ignore R1 because R1>>R2). That will definitely 
limit the response-time of the regulator. As you increase the load current, 
the converter will need to respond faster to replenish the output voltage 
(hint: I=Cdv/dt ). Your measurements clearly show the output voltage 
dropping at higher load current, so that's a definite clue.

Also, if you can run simulations that will help a lot. LT Spice (Linear 
Technology) is a great tool; I dont know if they have a competing part for 
the Max1771 (Maxim ?), or if Maxim has a similar tool.



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