MC34063A Poor load regulation in case of boost and buck converters
What could be the issue here ?


Look at figure 9 in the data sheet. There is a table indicating the line regulation is 30 mV from Vin = 8.0 V to 16 V, IO = 175 mA. Your Vin is 7.8 volts AND you are using a 220 ohm resistor in position R2 instead of the recommended 180 ohm. I would also suggest that your inductor is probably not good enough for the job.


  but with 5v input the voltage drops to 2.9v


Your expectations of it being a low dro-out buck regulator may be unfounded. Look at figure 11 - it is a 5 volt regulator and it specifies Vout from input voltages as low as 15 volts only.

It also appears that you haven't closely followed the PCB layout guidelines given on page 9.