I'm new to the game. I own the first edition, but hadn't really started getting into this until the second edition. Therefore, I am a raw recruit at all of this. But I am confused.
When I read what the Oxfordian Mage does, it allows you to split AP costs for spells. Okay, so you take extra time on your spells. Gotcha. That's pretty potent, especially when casting outside of Dramatic Time. Okay, I am good there.
Next, I read the Enhancement spell (mental or physical, it doesn't really matter), and it says it raises your stat by +1 to your Aspects (so that's +1 to all of them, nice!) to a max of 5. Okay, so 4s go to 5, 3s to 4, and so on. Cool. This lasts for 1 minute. Right. Range of 1 yard. Gotcha. Now, you want to increase it to a day...so that's 4 extra time steps, which is +8 TN. So now we're at a base TN of 18 to add +1 to all Aspects of one category or the other for 1 day. To get that TN remotely under control...say back to its TN of 10 or less, you're going to add +3 AP, lowering the TN to 9. The spell gets cast at the end of the second round and gives the target a nice boost for a full day. Groovy.
If you wanted to buff their same Aspect category a second time, you have to cast the spell a second time, which automatically ups the TN by another +3. So, you take a little longer to cast it, and boom. Cast.
So, walking through this, I get it. I see where things go wrong.
Its the Oxfordian boost. Its allowing you to use additional AP on a spell in a way that probably wasn't intended. As a ref of many many years, I would immediately argue against removing the ability. That's penalizing creative thinking and that's never okay. Instead, what about a sliding reduction in adding additional AP? First one lowers the TN by -3, second one by -2, third by -1, and fourth and beyond have no impact on the TN of the spell? This limits the added AP TN reduction to a maximum of -6, thereby eliminating the mega-stacking of a year's duration to a buff spell, or whatever.
Now, I am sure that veterans of the system will immediately see problems with this suggestion, but on a simple reading, that's where my alarms go off...the Oxfordian boost. Everything else actually works out due to escalating TN or the inability to stretch the AP beyond what can be done in a single round.