Exhaust gaskets keep blowing.

I did a fresh engine rebuild on my 1998 RMK 500 about 1 year ago, new pistons, sleeves, carb rebuilds, gas lines (no oil injector) etc. I rode it in Idaho for about 50miles, last year and it's been sitting turned on every week and ran for 10mins. We got snow where I live this year which never happens. I rode it for 40mins blew a exhaust gasket, replaced rode for 2hrs, blew a gasket again, rode for 4hrs blew a gasket again. I've blown 10 exhaust gaskets in 1 day about evenly split between both exhaust gaskets. The speed has varied widely between them blowing. I've cleaned and filed down the exhaust and outlet on the header. I've double check the bolts are going in nice and snug. I can't find a reason why there blowing ( I am riding at a low alltuide almost 400-500 lower than I normally do.) That's the only thing differnt I can find that I normally do.
Piston Readings: 140-150 per Piston.
Cold start: 2-3 Pulls (1/8 throttle choke)
Warm start: 1Pull (1/8 throttle)

Exhaust gaskets keep blowing.