Is always take three 25/100's of sinemet per day even if your doc says only two. It takes that much to get blood/ brain levels up to where your passing enough L-Dopa across into the brain.
I also would try an agonist as Carolyn and Birte said, but before you do that, try the magic Amantadine, it is synergistic with sinemet to a great level in many people. YOu can have side effects but often they go away, but it doesn't work for some.
Agonists are tricky because you have to titrate up slowly, and if you are working, and you turn out to be refractory to agonists, you could have a few very bad weeks.
Right from the start i had the symptoms you describe and i got my neuro to put me on Klonopin at a very low dose 0.5mg only when you feel really tight. 5mg of Flexeril often is useful, but can cause drowsiness, just as some people use benedryl. Ther are other muscle relaxants that cause little drowsiness, ask your doc.
BUT, give the Amantadine 2 X 100mg/day. good luck. cs