A few thoughts of mine that are mostly duplicates of what you have heard.
Power loss and momentary spiking of the gauge at the same time may indicate an engine ground problem. Electric gauge sending units work by altering the resistance through the ground wire. When the ground is poor, the resistance is altered.
Ignition systems also fire to ground, which happens to be the spark plug-a poor engine ground likewise will cause power loss, and it often occurs under load, because the engine twists against the motor mounts which then separate a bit by design. The mounts are steel bonded to rubber and bolted together. The bolt acts as the ground when the true grounds fail. Unfortunately they don't do a great job of it.
Look at the engine ground to chassis strap, it is going to look like bare flat braided wire. If it is corroded, rusted, loose or otherwise not making a solid connection fix this. I am inclined to believe this is your problem.
If your inspection does not reveal a problem here, check the condition of your radiator cap-it holds pressure at a set specification and when it is worn or incorrect your radiator's cooling capacity is severely reduced. I notice on your picture that someone added an overflow tank, but it appears not to be hooked up. This suggest the cap has probably been replaced at some point. Verify that it is correct for your vehicle.
Check the belt that drives the water pump, make sure it is snug and not slipping or sliding on the pulley. Water pumps fail-but they are easy to change and available inexpensively here in the states. It is a couple hours work or less. When they fail, they often leak through a small hole that is built into the housing. If this weep hole is draining you probably need a water pump.
Inspect the hoses for signs of collapsing or leaks, if they are swollen near the clamps it is time to replace them.
If you car is actually overheating (and it is just as likely that it isn't) and you have done all of the above, I would start with replacing the thermostat with a new and personally tested unit. Coolant should be flushed with engine running and the heater on and set to hot, until you get clear water and then drain completely and refill with 50% water/50%coolant mixture.
There should be a seal between the radiator and the AC condenser that you can make easily out of a scrap of any heat resistant plastic. (I cut down one from a 2000 Lincoln I bought at the junkyard for $3.00) This helps, but isn't always necessary. Also your condensor looks to be a little dirty and banged up. You can get a fin comb for straightening out the damaged fins and increase airflow more than you might imagine. I am unsure if your fan uses a clutch or is just a solid hub. If it has a clutch it will look like a finned plate between the fan and the water pump. If so, you can spin the fan and if if spins freely on a hot engine, it is bad.
Pay attention to the thermostat being a cleveland specific one as mentioned earlier-also if you remove it, there should be a small brass plate called a restrictor plate underneath it. If this is missing, it needs to be replaced.
Griffin makes a nice drop in radiator for our cars about $300.00 US
Milodon makes a nice aluminum high volume water pump for about $100.00 US A remanufactured iron pump will cost about 1/2 that amount.
If you need a water pump, then you need a new belt, to perform a good flush, fresh coolant, a new and correct and tested thermostat and a radiator cap. Factory radiators, if not leaking, are adequate to cool unmodified conditions in 95 degree weather with the AC running-unless your radiator is full of gunk that won't flush out or leaking or damaged, you probably don't need one.