Power loss, won't accelerate

Volvo 1983 240 DL. Well maintained. Given to young Chinese lady, can't drive stick. Parked under carport 3 years, started once/month. I am teaching owner to drive now. Car started easily, sl rough idle for 20 min, then ran well. Put carb cleaner in tank before driving. Ran fine for 30 min, then when engine and ambient air hot, began to lose power under load. Idle is fine. Going up sl hill, unable to accelerate over 2000 rpm, then after 10 min. Unable to accel on flat road over 1500 rpm. Idle is normal, accel when stopped rough, but will go to 3000. Shut off car for 5 min, then will accel to 5000 rpm when stopped, but won't run for 3 min. Until problem occurs again.
Next day, ran for 40 miles on freeway with no problem. Cool morning 50 F. At noon, temp. 90 F, and same problem after driving in town for 15 min.
Owner has very little money, I work for free.
Thank you so much