rhotacize

from The Century Dictionary.

• To use r too frequently.
• To make wrong use of r; pronounce r with vibration of the uvula instead of the tip of the tongue.
• To convert other sounds, as s, into r; substitute r in pronunciation.
• Also spelled rhotacise, rotacize, rotacise.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb To pronounce with an exaggerated 'R' sound (a rhotacism).
• verb linguistics The change of a consonant to an 'R' in certain circumstances.