MKI power problems

I own an 88 mkI N/A 5 speed MR2. When i purchased the car in july it ran like a champ i mean it had balls. then after about 200 mi of driving it after i bought it it would loose a signifigant amount of power when it reached it's normal operating temp. But had it's regular amount of power while the engine was still cold and it has been like that for quite a while until recently when i have had a lack of power all the time. here's some more info about it: about three months after i bought it i think i ran outta of coolant cause i was driving like crap wouldn't get above 2000rpms luckily only drove it about a half mi, filled up coolant and it was running like it was before again. i have since replaced the timing belt, clutch, and O2 sensor. recently when bleeding the cooling system i noticed the coolant coming out from the radiator bleeder valve was milky the heater bleeder valve though the coolant was fine. i haven't tested compression yet cause when doing the test my compression tester decided to break when testing the third cylinder however the first two got 155psi which isn't great but above the minimum.

Head Gasket? maybe? fuel pump, filter? I only have a basic mechanical knowledge so my trouble shooting just goes to the obvious things.

Really what i am asking for is some ways to help me diagnois my problem unless someone thinks they know the problem.

First, you need to check the compression on the other cylinders. Also, a blown headgasket should be obvious by the condition of your sparkplugs. If they read OK, I'd next check with the ignition timing. Make sure your base is correct and check to see if the ECU is putting the advance where it's supposed to be.