Sounds like the slow speed escapement needs to be cleaned.
I had to clean the same on a Nikon F2, but have never cleaned the one on a Minolta SRT.
Here's a video that addresses that issue.
Edit: One note. Interestingly enough, the position of the escapement on the F2 was as critical as it is on a leaf shutter camera. I scribed around the escapement before I removed it to get it back into the same place, but still had to fiddle a bit with it to get speeds closer to those listed on the shutter dial. Don't know if this applies, but I would scribe and measure from some point inside the camera body to get it back in the same general location.