parked at tuscon airport for a week, temp in 90's, driving on freeway heard loud clunk in first 5 minutes. about 15 mins later heard same clunk within a minute of each other. all lights went off for split second both times. stopped at next town. no more symptoms. this happened about 1 year ago. what should i fix ?

not gonna happen. we live in the mountains and the nearest volvo shop is 185 miles away in elpaso, . my best guess is its temperature related and the clunk was the transmission control re-engaging at freeway speed. its not happening in my town, because it hasn't gotten hot here. would the e.c.u have a code for an internal fault ?

sorry i should have been more clear. the first incident happened a year ago with the car parked at the hot tuscon airport and that time the air conditioner kicked off, dash lights and electric windows went out . and wouldn't run while the car was on the freeway. at the next exit was a mechanic who disconnected the battery to reset the computer and all was fine for the remainder to the 200 mi trip in the heat of the summer . the latest 3 thumps were yesterday evening on the trip from the tuscon airport.

More than likely the shifting solonoid 1-2,2-3 are sticking. You may get a "trains service required warning " message , Egine service light , and limp home in 3rd gear. Reset the service code and you should be good to go - until it happens again. Solonoid replacement is the permanent cure about $1000 job. A weak point of volvos from about 2001 on. Driving in the W mode does reduce frequency of of the problem but may have additional consequences.

yachtboy, thanks for your answer. many electrical items in the car also ceased functioning. at the last event , the clunk and other electrical outages occurred 3 to 4 times so we pulled off the freeway, raised the bonnet waited awhile and then continued on our way with no more problem which is now 6 months later and many long journeys later. the only 2 days this problem arose was upon arrival at the Tuscon airport parking on a hot day, late afternoon.