Most likely cause abrupt onset of misfiring at idle less at higher RPMs

What is your question?

misfires more at low RPMs engine continues to skip and misfire while accelerating Skip's less at at higher RPMs problem started with 87 gas for first time back to 93 but misfiring continue to increase problem began suddenly no apparent cause. No oil leak no overheating no warning lights only check engine as if it had a wet distributor cap. Added fuel system treatment when problem began problem continued to deteriorate. Checked all fuses checked air filter oil also fine. Began almost unnoticeable and increased steadily. Misfiring increased quickly over 2 days. No other apparent problems. Core had always run great until this point.

What makes this problem better or worse?

Misfiring is much worse at low rpm misfiring infrequent as accelerating and at top end

How long has your 2000 Volvo S40 had this problem?

Problem started after using 87 gas for the first time returned to 93 but still persists continuing two misfire more frequently from beginning of infrequent misfires song problems with idle too frequent misfire only 2 days