Aprilia sr50 cuts out ?

Hi
I've got a 2009 Aprilia sr50 (piaggio engine)
with just over 8,000miles on it
I've recently fitted a stage6 streetrace 70cc kit with malossi reeds (I've thredlocked the screws that hold the petals in the block just to b on safe side)
i've also fitted it with a brand new small end bearing and a 72 mainjet in the standard 17.5 delorto carb with standard airbox
brand new sparkplug wen removed has a chocolate brown/coffee colour with I beleive to b good?
Compression tested it and got 130psi
basicly wen riding for long periods of time it suddenly cuts out and lights go out instantly but it will start straight up again and ride perfectly
I don't think it's a reed issue bcoz it starts up first kick and will tick over ten to 15mins no problems

it don't overheat as it's at 3 bars like it woz wen woz a 50
I don't kno if it's related to the accasional cutting out but wen changing the rollers I saw that the left hand crankseal is leaking?
I'm ordering 1 in next few days just wondered if it's easy to swap and any tips to do it correctly
and anything else to check regarding the cutting out
thanks for taking the time to read this and hope someone can help
don't kno if it helps but the topend has less then 20miles on it
and on both times it's cutout it's been raining