1990 740 hesitates in heat

[stauda]

I have a 90 740 with 155000 miles I have owned for about a year. I noticed this problem early-on and it seems to be worsening.....When driving at higher speeds(65-75mph) the car has slight hesitation/bogging during deceleration..most noticeable when I am going downhill and using partial throttle to maintain speed. The problem seems to be worse in high temperatures. The car finally had a stall which I believe was due to this problem which left my girlfriend stranded(temporarilly...car restarted and ran fine after 15 minutes) in 110 degree heat last week. Any suggestions? My local volvo mechanic is stumped