I have an 08 maxima an my check engine light is flashing, so far I changed the spark plugs and coil packs after doing so my raidator cracked so I had to get a new also. My car also starts shaking around 1500 rpm and 40 mph but if I go over 2000 rpm and over 50 it stops shaking but the check engine lights keeps flashing

 

Post your codes. Balance your wheels. Replace your axles.

 

My tires are new an balanced and the code is P0300 and I lose power and car shakes around 40 to 50 and 1500 rpm and check engine light starts to flash

 

A CEL that is flashing means that one or more of your fuel injectors is malfunctioning and too much fuel is being dumped. Per the FSM, the vehicle should not be driven when this happens.

0300 is a generic code for misfire. You've replaced the spark plugs (hopefully with NGK's), replaced the coilpacks (hopefully with OEM), all that is left is the injectors.

Clear the code and check again. With any luck, you'll get a specific cylinder code like 0301, 0302 etc