I doubt it's the solder, itself. Even a low MP solder will only melt at DOUBLE of what your motors should be temping out at if geared even optimistically. Considering you have been using the same stuff for 4 years, it's likely that you would have had issues before.
Cold solders are more likely. Dirty connectors, bad soldering practices, no flux, non R/C ESC wire and wires too short could all be problems.
I wouldn't even mess with anything in this case. Replace ALL of the ESC wire to the motor with good quality 12g silicone wire sold for R/C racing, use brand new SP motor plugs/connectors, tin the wires before soldering together, use a good quality flux on your connection, and use the proper capacitor for the ESC.
I would also open up a cooling vent in the body for the motor, and use a temp gauge to monitor the temps of the motor on your longer runs—just to make sure. Solder melts usually over 200° C and motors shouldn't be geared to be above about 80° C, so you're well out of the range for heat issues. Bad, dirty or cold solder joint on the plugs is likely your culprit, but you should try to monitor and limit the heat to your motor anyway, if possible.
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