oli problem

i have a 86 944 ,non turbo that has a problem that i have been trying to sort out for 4 months with no success. when the car is fully warmed up when going down the road there will be a massive cloud of smoke[blue] out the rear of the car for maybe 10 seconds and then just disappear and may not happen again for maybe 2 weeks. it almost seems that something is letting loose and rresealin just as quick. i have not had anyone follow me to determine if it is from the taailpipe or is being burnt on the exhaust . any help is appreciated sam

i should mention that i am leaning towards a head gasket but there is no sign of oil contamination and do not know how it would cause the smoke. the engine other than this runs strong sam