Hi lamma,
I understand it could be upsetting not to be able to use the computer properly.
If I understand you correctly, you mean to say that you occasionally hear 2 beeps from the system. Is that correct?
2 beeps for this system model means No Memory (RAM) detected. The possible cause of failure could be Memory or the Memory slot.
I would suggest you to run the Dell Diagnostics to check the hardware functionality. Power off the computer -> restart the computer, press 'F12' key on the Dell logo -> Select ‘Diagnostics’ from the boot menu. Please share the results.
I would also suggest you to re-seat the Memory of the computer. Please refer to the Service Manual.
Please reply with the findings; will take it further.