Loss compression my car has not strong enought since it was repair after overheating

How to Test Engine CompressionAn engine depends on an equal compression reading in each cylinder to run smoothly. If poor compression exits in one or more cylinders it can cause a rough idle condition and low power. A compression test can be performed to check wear or internal damage. To start a engine compression test gauge is needed. There are two types of gauge styles, one threads into the spark plug hole which is more accurate. The other style of gauge is constructed with a rubber plug that is meant to be pressed against the spark plug hole, this style of gauge is difficult to use. To perform a engine cylinder compression test follow the steps below.

• Remove ignition coil connector or ignition system fuse to disable power to the ignition system
  
• Test ignition system to ensure that the power to the system has been disabled
  
• Remove #1 cylinder spark plug and insert compression gauge, most gauges have a hose attachment that is installed into the spark plug hole with the gauge connecting to the gauge hose.

• After the compression gauge has been inserted, use the starter to crank the engine over for about five seconds. Use about the same five seconds to test the remaining cylinders.
  
• Record the compression reading as each cylinder is tested
  
• Remove the compression gauge and reinsert the spark plug
  
• Follow this procedure until all cylinders have been tested
  
• Compare cylinders compression reading, all readings should be within about 5% of each other

Loss of compression while driving at speeds of 75-80 mph causing the rpm to go down while accelerating. What could this be.. Just got new heads done on engine.