Sounds like a lot of first jams, most importantly you learned a lot. For one, all jamming kind of have to know where the other person is going next or a jam will not sound good or feel good to play.
For working on your timing I have two ideas.
Anders Mouridsen teaches La Grange. Check out the Have Mercy: First Solo Rhythm.
http://www.guitartricks.com/tutorial.php?input=1452
It’s a great easy rhythm and you can force yourself to play as slow as he does jamming along. Match him strum per strum, I’m not talking about the lead, just the rhythm. Plus, La Grange is kind of like Smoke on the water. Every guitar player should know it. Guaranteed to get a crowd on its feet.
Metronomes are great, but how are you going to keep a beat if one is not around and a guitar is. Tap your foot 1 2 3 4 1 2 3 4 bounce your leg, get more of your body evolved so you can feel the tempo trying to change.
Being aware of the problem will help a lot.
