1994 850 turbo surging

[palebird]

I have a 1984 850 turbo sportwagon and it surges when I take it out on the highway and it is still relatively cold..the temp gauge is reading normal but it has only been running 10 minutes max. Sitting at 110 km there is a significant surging feeling although I can't really notice anything on speedo, tach or boost gauges..almost like the brakes are being applied..weird..it goes away after about 5-10 km and will not come back unless the car is left to cool down for a few hours..any ideas?