You do need to be careful here with the hair drier! Depending on the unit it could damage your system with either ESD or poor grounding.
Hair driers are not quite powerful enough to push the deep dust, nor can they be focused as fine a point as compressed air (can'ed air) can. Compressed air also does not create ESD.
I would open up the system again and use a soft paint brush scrub down any remaining dusty areas and use can'ed air to push away the lat bit of dust.
Check your connections you may have a loose wire. Also this systems power supply has been known to go. Look at the capacitors do you see any wet areas around there bases and/or see the tops bulging up a bit. That would be a sign of a cap that is failing.
Given the age of the system I would also refresh the thermal paste.
But before you go to far here, consider the costs and what you use the system for. It maybe time for a new system.