What services have been done? There's about a bazillion different things that can cause rough idle. Your car can have codes retireved by an impulse counter the HHT is not necessary. The electronics on your car are not that sophisticated, meaning that if there isn't a fault light on a code check won't reveal anything.
The blower could be just a faulty switch.
92 Volvo 740 Wagon aka "Mutt mover"