My 91 Cadillac DeVille after driving so long or it's hot, starts acting up

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After driving about 40 minutes or if it's too hot outside, my 91 Cadillac DeVille will start acting like it's out of gas, the acceleration will be weak and soon it' die, after sitting turn off for a while it'll start up fine then a few minutes later it'll the same thing

How long has your 1991 Cadillac DeVille had this problem?

Since March