Boot Loop with random horizontal colored line after apple logo

Did you remove the shields when you cleaned the board? If not, most of the board is actually hidden from view and that’s where most of the water damage occurs.

If you did and the main power rails look okay, I would start by going "bare bones". Disconnect everything from the logic board except the Battery and Dock. Connect an iTunes enabled computer and run 3uTools (www.3u.com) to see if the phone is still rebooting. If it is, then it is most likely a logic board issue (it could be the dock though).

If you suspect a logic board issue, check PP_BATT_VCC, PP_VCC_MAIN and PP5V0_USB. I would start by checking to see if those rails are shorted to ground (which you already did!?) . If one of these rails is shorted to ground, then you will need to identify what is causing the short. It could be a bad decoupling capacitor, conductive debris or defective IC that is directly supplied by those rails.  Also look at whatever specific circuits were attacked by water or corrosion.

Then you move onto the PMIC and check the voltage rails it generates:

• The PMIC generates ~15 voltage rails. They are all important (for obvious reasons) but the ones to check first are as follows: PP_CPU, PP_GPU, PP1V8_SDRAM, PP1V2_SDRAM, PP_VAR_SOC, PP0V95_FIXED_SOC, PP3V0_Tristar, PP3V0_NAND, PP1V8_ALWAYS and PP1V0.

• The PMIC also generates, what I would consider secondary, yet still important voltage rails for the following sub-systems: PP3V0_MESA, PP1V8_VA_L19_L67, PP3V0_PROX_ALS, PP3V0_PROX_IRLED, PP3V0_IMU, PP3V3_USB and PP3V3_ACC

This should give you some clue to where the problem is. Report back with your findings.