Rhythmic variation is an excellent way to change up the process of learning difficult sequences. Altering the rhythm is certainly related to grouping variation, but it is also possible to decrease (or increase) the rate of notes, or to move the emphasis within the meter. Here are a few possibilities for your next practice session.
This is fairly self-explanatory. If the passage is in 4/4, for example, change the meter to compound time. The beat moves, but the proportional rhythm stays the same. In other words, change eighth notes to triplets, or vice versa.
Ma-ry / had a / li-ttle / lamb
Ma-ry had / a li-ttle / lamb
In a passage that combines complex sequences of notes with tricky rhythms, try just playing the notes in straight eighth notes. By eliminating the rhythmic complexity, the focus shifts directly to getting all the right notes. Start slowly, and when the tempo reaches, or slightly surpasses the fastest subdivision of the goal, return to the original rhythm. This same trick works in all triplets, groups of five, or any other static grouping.
This is a standard practice method used in etude and study books. Say that the original rhythm is eight notes. Go through a series of variations that explores the many ways that two notes can be spread across a single beat. Two eighth notes could become dotted eighth + sixteenth, sixteenth + dotted eighth, quarter note triplet + eighth note triplet (or the reverse), swing eighth notes, or anything else that you can dream up. Make up your own series of rhythmic variations and make it part of your routine.
These ideas are all very simple, but the concepts are widely applicable. Scales and patterns could be practiced in long chains of different rhythms. Try writing short rhythmic figures on slips of paper, shuffling them up, and then assembling a random series. We are only limited by our own imaginations.