Tom Beim was the worst offender when he dropped Fanolua' s pass on the 22-metre line with a clear run to the posts.
The freezing temperature hardly helped handling, but when the winger fumbled again minutes later the Kingsholm faithful were just as unforgiving to their top try scorer. Mannix's kicking went astray as he missed three routine penalty chances while Northampton full back Grayson kicked three himself.
Grayson's fourth penalty put the Saints 12-0 ahead before Gloucester finally broke through in first-half injury time when Fanolua raced over from Olivier Azam's long pass. The Samoan centre converted his try to leave Northampton with a 12- 7 half- time lead.
Gloucester bombarded the Northampton line at the start of the second half and tied it up at 12-12 when Mannix followed up Fanolua's break and ducked his way between two tackles to squirm over the line.
Roared on by a crowd of 9,859 - their best of the season - Gloucester cranked up the pressure but Northampton held them out and Grayson landed the winner with 15 minutes left.