I agree that checking/cleaning the distributor cap is excellent advice. Possibly a new distributor cap carbon brush & spring might be a good idea also. I cannot see the application of WD40 on a upside-down mounted distributor cap to be helpful.
Carburetor float levels and a stretched fuel pump diaphragm would be two other causes.
The timing belt is not adjustable for tension, and ½ inch does seem a little high, so a new belt may be needed. If it is the original timing belt, it is due. The two sealed ball bearings (which support the rotor shaft) should be considered also (if original), or a new belt will typically break when a bearing(s) binds.
Regards,
Joe
(770) 597-5992
