Hi Adam,
I don't think its mainly a priority problem but more a init problem of your timer0.
Do you use T1 for baudrate generation ?

If so make shure you dont change the T1 bits when you are changing T0.

Leaving all IP bits at zero causes the priority as descriped in the datasheet

Prioority from highest to lowest:
EX0->T0->EX1->T1->Ser->T2

Hope this helps