Mardy...
Odin's right in his suggestion above. There's a good chance that you have a back- pressure problem. Having problems a few times related to motor longevity, even when the motor is well- oiled, indicates that something the motor is dealing with is causing problems with the motor. Repairing the motor addresses a symptom... not the cause. My guess is that the silencer is the culprit.
It may be that something as simple as lowering the motor RPM (which also lowers the blower's and therefore the potential back pressure...) is a remedy.
The best thing to do is to get a 5 PSI pressure gauge and connect it to the motor manifold. (Get someone to weld a fitting onto the manifold for that purpose. Use a long copper tube to connect to, so that you can deal with the heat.) My Briggs rep told me around 1999, that the maximum PSI allowed is 3 PSI. (Kohler recently told me 1.4 PSI, so it seems that they're different, maybe more for emissions than longevity.)
If you take the motor off to work on it, be careful not to over- tighten the belt tensioner as well, when you put it back on the frame and connect everything. Those pulleys are pretty small and close together (so a larger size can't be used there) and you need to run a pretty stiff belt to compensate, but there's not much room there to play with. If they're too tight, you'll have problems with the shaft seals in the rear of the motor.
Duane
