Inspiron G3 15 3590, behaving abnormally

Due to overheating I just got its heatsink and fans replaced. 

However, when I started my computer again after 4-5 hours later, system started behaving abnormally - sluggish performance, games are getting very low frame rates etc etc.

When checked power settings on Alienware Command Center, I saw that power is set to performance and thermals to performance as well, but clock speed moving around 590MHz and 0.9MHz when playing games! 

And i7-9750H has minimum clock speed of 700 something MHz!

Moreover, whenever trying to restart the system, it shuts down! I thought Windows might be an Issue, but nope  even when rebooting system after installing windows initial phase it shut down. 

PS: The technician that replaced the heatsink, used a screwdriver to chop off heatsink thermal pad that was bulging out of heatsink after tightening of screws to motherboard.