If it is beeping during boot up there is a problem if there are more than one beeps. If there are no beeps then that is also a problem likely to be Motherboard or memory. 1 Beep is normal. If you have several beeps you will need to count them then look them up in a table appropriate to the motherboard chipset.
I am no expert in this area it is just that I recently read about beep codes when trying to work out what was wrong with a PC, (it turned out that the reset button had jammed partially in).
You will need an expert to interpret the beep code but as you have an intermittent problem i would suggest that either a component is on the way out or the unit is overheating.