Front heater fan switch

I own a 1991 Pontiac Transport,it has had very few problems but is has one that is puzzling,my heater fan seems to work on it's own,at times it works on all three speeds and sometimes it only works on high and then the next time it won't work atall!!!
i am thinking it may be a relay overheating but if anyone else has any ideas please let me know.

THis is the diagram
www.avigex.ca/xport/hvac_diagram_front.jpg
the low speed relay feeds the blower at all speeds except HIGH, where it feeds the high speed relay coil current only. I'd first check the low speed relay, possibly a poor contact that is capable at times to feed only the small current needed for the high speed relay coil ( Hi speed relay power contacts are fed directly from the 12v bus). The low speed relay is in the convenience center
http://www.avigex.ca/xport/convenience_center.jpgand for a test you can switch it with the horn relay just beside that is similar.

I own a 1991 Pontiac Transport,it has had very few problems but is has one that is puzzling,my heater fan seems to work on it's own,at times it works on all three speeds and sometimes it only works on high and then the next time it won't work atall!!!
i am thinking it may be a relay overheating but if anyone else has any ideas please let me know.

I had this same problem before and it was the blower motor. The bearings had worn and the armature was making contact inside the motor not allowing it to rotate, a bump or a jar would let it rotate and work again. The cure was a new blower motor. 5 screws and plug on the engine side of the firewall. I believe it was under $20