Gohan never completely reached SS2 before cell, and therefor lost the complete control of the transformation he had in FPSS form. Just like how Goku went pretty mental when he did his first SS transformation.
IIRC, the FPSS "form" isn't a form, but rather a state within the normal SS transformation where the saiyan is calm and got full control over himself. This way he can control his energy consumption much better and think clearer. There is no screwing around when they go FPSS, since they think and act like in their base form. But when Gohan went SS2, he was like a completely different person, all angry and vengeful.
Goku had so much control of SS2 from the start since he had probably used it several times while dead. IIRC he could use SS3 while fighting SS2 vegeta, which leads me to believe that he had used SS2 a few times before and therefor could control it.
Vegeta had the hard power increase from babidis "control" which i think might've helped him control the form. (i think it is stated at some point that vegeta went SS2 only because of babidi, which is why he let babidi "control" him)
Sorry i ramble without much sense, but im tired as fuck.