In the situation you are in, the only objective resolution is to rehearse as a band with a LOUD drum machine / metronome playing. This is the only way to REALLY find out who is speeding up, who isn't, and who has a good sense of time. I have advocated practicing as a band like this for a long time. However, it isn't the easy to convince other, less rhythmically supportive, members to practice that way. Its challenging, but it teaches other members of the band (perhaps that don't have the experience you have with a metronome/drum machine) to LISTEN to the beat as opposed to neglect it during a solo.
Thoughts on this?
This is a good idea, I'm thinking. It might be hard to sell if your group has a drummer and you're not him, but if there is no drummerlike in my groupI think it would be a much easier sell. I'll see if my group will buy in.