Macbook works fine, but when hot, it is lagging really bad!

Hi everyone!

Ok i am really stuck at the moment. When i use my macbook after a while it starts to lag when im runnning a program or web browser, such as youtube for example. It is really annoying! I recently updated my macbooks 4gb ram to 8gb and replaced the battery too. Did the person who fixed my laptop do something wrong? Is it something i can fix myself? I tried the PRAM and SMC reset and no luck.

My Specs:

Macbook 13inch A1342 Late 2009 320 Hard drive only 100gb of it used....

Processor 2.26 GHz Intel Core 2 Duo Memory 8 GB 1333 MHz DDR3 Graphics NVIDIA GeForce 9400M 256 MB

Software OS X 10.9.1 (13B42)

But before i had the 4gb ram.....

Is the ram too fast for this computer?

Also my battery says "Service Battery" but it is brand new:

Charge Information:

Charge Remaining (mAh): 3049

Fully Charged: No

Charging: No

Full Charge Capacity (mAh): 3967

Health Information:

Cycle Count: 33

Condition: Service Battery

Battery Installed: Yes

Amperage (mA): -1084

Voltage (mV): 11513

Some Please Help! Thank you!