Are you cleaning the boards after soldering? From what I understand, you must clean flux residue from water soluble flux.
From the datasheet of the AIM 13154 solder:
OAJ flux residues MUST be removed after soldering.
The solder you purchased uses OAJ flux.
From what I have read, OAJ flux residue has a lower resistance than normal flux residue. The lower resistance can cause high impedance circuits to misbehave.
OAJ flux is also more aggressive - the residue can apparently attack the copper and damage the connections over time.
Additionally, AIM says to use higher soldering iron temperatures than you would normally expect for solder with lead:
Solder iron tip temperature should be between 350° - 400°C (650° - 750°F) for Sn63, Sn62 and Sn60 alloys, 370° - 425°C (700° - 800°F) for SN100C®, Sn/Ag and Sn/Ag/Cu (SAC305, SAC405, CASTIN, etc.) alloys.
I normally solder 60/40 tin/lead (the alloy you are using) at about 270°C. The temperatures AIM gives for 60/40 tin/lead are what I would use for lead free alloys. The temperatures they give for the lead free alloys are higher still.
- Use the recommended temperature.
- Clean your boards and joints after soldering.
I could not find an explanation for "OAJ." AIM says it is an "amine neutralized halide-activator system." I have no idea how you get from that to "OAJ."
I found an old post (from 2004) on the Microchip forum where someone has the same problem, and the same ultimate cause - OAJ flux that wasn't washed off after soldering.
The flux residue caused erratic operation, and caused known good circuits to misbehave.
Use hot water to clean off the residue.