If your radiator is the original one, go for an overhaul. Its about 1000 to 1500 pesos only. The refill with 50/50 mix of distilled water and coolant. Dont forget to burp the car. While your at it, check the t-stat and replace of needed. Do the same with the water pump, rad hoses, fans and v-belts.

Flushing is good but it wont clean out your rad 100%. A overhaul should also inc checking and fixing any minor leaks.

A grounding kit should help your car. You can diy or get a shop to do it. Use real 8 awg wire. dont get those that comes in its, they are usually undersized. Or better yet, go for a big 3 upgrade. The big three upgrades your bat neg to ground, engine block to ground and alt to bat + wires with 4 awg or bigger wire. The big 3 should cost about 1500 pesos. No idea how much for grounding kit. Either way, you need to clean and remove any paint from the grounding point.

Speedyfix is a very good shop. if he is taking care of your car, your in good hands.